Multiple receptor responses: A new concept to describe the relationship between pharmacological effects and pharmacokinetics of a drug: Studies on clonidine in the rat and cat

The time course of an observed pharmacological effect is affected not only by the kinetics of the drug levels at the site of action but also by parameters such as the slope and maximum effect of the functional relationship between drug level and response. Using clonidine as a test drug, it was found that the kinetics of its effects on blood pressure and pain responses cannot be described by the time course of clonidine levels in the blood, brain, or the hypothetical tissue compartment of the two-compartment characteristics of this drug. However, the results can be explained assuming that the observed pharmacological effects of a drug are composed of the sum of responses from at least two receptor sites with different slopes and maximal effects. The effect of intravenously administered clonidine on blood pressure in the rat was found to be related to the blood concentrations at least at two receptor sites with opposite effects, one leading to a hypertensive and the other to a hypotensive response. Predictions indicate that a maximum decrease of arterial blood pressure is obtained when the steady-state blood concentration of clonidine is about 1 ng/ml and that no effect is seen at 10 ng/ml. Higher levels will produce an increase of the pressure. The kinetics of the analgesic effect of clonidine in the rat could best be related to the brain levels if the observed effect was considered to be derived from the sum of activity at two receptor sites each producing analgesia. The kinetics of the effects of clonidine on the nictitating membrane of the cat was found to be determined by the kinetics of the drug in the peripheral compartment of the two-compartment open model. Consideration of multiple receptor responses is suggested for future studies on the relationship between the kinetics of drug levels and pharmacological responses.     

Pharmacokinetic and concentration-effect relationships of clonidine in essential hypertension

Summary The effect of oral doses of 300 µg of clonidine hydrochloride on blood pressure, sedation and saliva production in 5 essential hypertensives were qualitatively similar to the effects in normotensive subjects. Peak plasma clonidine concentration (1.34±0.28 ng/ml) and plasma half-life (10.0 ±0.8 h) were similar to normotensives. During chronic oral dosing there was no evidence of drug accumulation. Some tolerance to the sedative and salivary flow effects occurred but no tolerance to the hypotensive effect was observed. There was a linear relationship between reduction in saliva flow and plasma levels of clonidine. The hypotensive effect was also related to plasma level at low concentrations. At plasma levels >1.5 ng/ml the hypotensive effect was diminished. This loss of effect at high plasma concentration may be related to the peripheral, post-synaptic alpha-adrenoceptor agonist action of the drug.     

New aspects of the pharmacokinetics and pharmacodynamics of clonidine in man

Using considerably improved analytical methods, the kinetics and effects of clonidine were observed in healthy volunteers over periods of time more than 3 times longer than those previously reported. The high sensitivity and small work load of the newly developed method permitted the performance of low-dose and multipledose trials. 1. The complete bioavailability of clonidine and its elimination half-life (20 to 25.5 h) remained constant after single and multiple doses. 2. Approximately 62% of a given dose was excreted unchanged in the urine, independent of the quantity administered (0.075, 0.15, 0.2, 0.25 or 0.3 mg), the drug formulation (solution, tablet, Perlonget) or of the mode of administration (i.v., p.o.; single or multiple doses). 3. As the pharmacokinetics of the drug were affected by entero-hepatic circulation, it cannot be described by a conventional, open one or two compartment model. 4. The time courses of the plasma clonidine concentration and its drug effects ran asynchronously. 5. On cessation of chronic clonidine administration, blood pressure and plasma catecholamine levels increased to pretreatment levels without exhibiting any "overshoot" reaction.     

Pharmacokinetics of clonidine and its relation to the hypotensive effect in patients.

1 The kinetics of clonidine and its relation to the blood pressure response after single intravenous doses of 75 micrograms--275 micrograms in hypertensive patients were determined. 2 Clonidine disposition could be described by a two compartment open model and pharmacokinetic parameters show a rapid distribution phase of 20--30 min and a mean plasma clearance of 4.6 ml min-1 kg-1 (75--200 microgram). The half-life of the beta-phase was found to be in the range of 7.4--11.4 h. Indications of dose dependent kinetics were obtained. 3 A dose-dependent decrease in blood pressure was obtained. 4 The maximal reduction in MAP (mean arterial blood pressure) was significantly (P less than 0.01) related to plasma concentrations of clonidine. 5 The reduction in MAP was always related to plasma concentrations of clonidine (r = 0.88, P less than 0.01) when pseudoequilibrium of distribution of the drug was achieved.     

Pharmacokinetics of clonidine in the rat and cat

To investigate the pharmacokinetic behavior of clonidine, rats were given clonidine intravenously at 125, 250, and 500 micrograms/kg and blood clonidine concentrations were followed for 6 hr. The disposition of clonidine in two brain regions was studied in rats after an i.v. dose of 500 micrograms/kg. The liver clearance in rats was investigated by liver perfusion techniques. The results obtained indicate that the disposition characteristics of clonidine can be described by a two-compartment open model in both rats and cats. The penetration of clonidine into tissues is rapid, and brain levels in rats were about 1.7 times higher than blood levels. Brain tissues were found to be an indistinguisible part of the central (blood) compartment. Dose-dependent pharmacokinetic behavior was found for clonidine in rats at the doses used. This was demonstrated by a decrease of both the rate constant of distribution to the peripheral compartment and the overall elimination rate constant from the body, with increase in dose. As a consequence, the volume of distribution and the clearance both decreased with increasing dose. Possible explanations for the dose-dependent behavior of clonidine are discussed.     

The factors influencing the time course of drug action at alpha-adrenoceptors: an investigation of the effects of clonidine in the pithed rat.

1 The time courses of the pre- and post-junctional effects of clonidine were examined on heart rate and blood pressure and on the isometric tension responses of the vas deferens and anococcygeus in situ in the pithed rat. Plasma levels of clonidine were also monitored. 2 The time courses of the pre-junctional inhibition of sympathetic nerve mediated responses in all tissues monitored were related to the plasma levels of clonidine, but the degree of inhibition which was observed varied with the tissue and with the test stimulus employed. 3 The post-junctional agonist effects of clonidine, expecially at low doses, exhibited an initial 'peak' followed by a decline to a lower plateau. This decline of the response was not due to receptor desensitization but was related to a decline in the plasma clonidine level from an initially high value caused by injection of a bolus. 4 The factors determining the time course of drugs' effects in the preparation are discussed and it is concluded that pre- and post-junctional responses should be compared at a point in time after injection of the-drug, at which equilibration has occurred.     

Behavioral and physiological detection of classically-conditioned blood pressure reduction

Spontaneously hypertensive (SH) rats were trained to discriminate the effects of saline injection from the interoceptive stimuli associated with the blood-pressure-reducing effect of clonidine (0.02 mg/kg, IP) in a drug discrimination procedure. Anise/ethanol and ethanol odors were then systematically paired with clonidine and saline treatment, respectively, outside the drug discrimination setting. As the number of pairings increased, the anise/ethanol (but not the ethanol) stimulus, when given alone, came to both reduce blood pressure and to mimic clonidine's interoceptive stimulus to virtually the same extent as clonidine itself. Both responses induced by the conditioned stimulus (CS+; anise/ethanol odor) were antagonized by the noradrenergic alpha-2 receptor antagonist yohimbine at a dose that did not by itself influence blood pressure. These data support the hypothesis that activation of endogenous factors can be elicited by a CS, and that these factors may furthermore act agonistically at central alpha-2 receptors to reduce blood pressure in hypertensive animals.     

An integrated approach for the evaluation of psychotropic drug in man

Following an integrated approach based on the contemporary recording of drug plasma levels, central (CNS) and peripheral responses and performance tests, the effects of two different amphetamine formulations were evaluated in healthy volunteers.Interesting relationships were observed between amphetamine initial rate of entry into the blood stream and both incidence of side effects (S.E.) and rise in arterial blood pressure. Minimal thresholds for CNS (5 ng/ml) and peripheral (20 ng/ml) effects could also be determined. It appeared also that the personality of the subject may have some bearing on both incidence of side effects and performance. The described methodology seems valuable in valuating psychotropic drug effects in man through a comprehensive integrated approach.     

Pharmacokinetics of clonidine in the rat and cat

To investigate the pharmacokinetic behavior of clonidine, rats were given clonidine intravenously at 125, 250, and 500g/kg and blood clonidine concentrations were followed for 6 hr. The disposition of clonidine in two brain regions was studied in rats after an i. v. dose of 500 g/kg. The liver clearance in rats was investigated by liver perfusion techniques. The results obtained indicate that the disposition characteristics of clonidine can be described by a two-compartment open model in both rats and cats. The penetration of clonidine into tissues is rapid, and brain levels in rats were about 1.7 times higher than blood levels. Brain tissues were found to be an indistinguisible part of the central (blood) compartment. Dose-dependent pharmacokinetic behavior was found for clonidine in rats at the doses used. This was demonstrated by a decrease of both the rate constant of distribution to the peripheral compartment and the overall elimination rate constant from the body, with increase in dose. As a consequence, the volume of distribution and the clearance both decreased with increasing dose. Possible explanations for the dose-dependent behavior of clonidine are discussed.     

Fluorocarbon aerosol propellants XII: Correlation of blood level of trichloromonofluoromethane to cardiovascular and respiratory responses in anesthetized dogs.

Anesthetized mongrel dogs were exposed to various concentrations of trichloromonofluoromethane. Before, during, and after the inhalation, arterial and venous blood samples were obtained for fluorocarbon analysis. After the cessation of fluorocarbon inhalation, a multiexponential decline for the blood was observed. This finding was similar to that of previous study in which the fluorocarbon was administered intravenously to unanesthetized dogs. The half-life calculated from the terminal phase was about 280 min, and the pseudodistribution equilibrium was reached about 100 min after dosing. Study of the relationship between blood fluorocarbon levels and effects on the respiration rate and arterial blood pressure indicates that the sites of these pharmacological activities are located in the blood or central compartment rather than in the peripheral compartment. The effect on the heart rate appears to be quite instantaneous after inhalation. These results might shed some light on the fast effect of the fluorocarbon propellants, which caused sudden deaths after inhalation of a large quantity.     

Relationship between the cardiovascular effects and steady-state kinetics of clonidine in hypertension

Summary Clonidine was given orally as monotherapy in increasing daily doses from 3.1 to 25.7 µg/kg to patients with essential hypertension (n=6). When a steady state concentration in plasma was reached at each dose level, the blood pressure (BP) and heart rate were measured during a dosage interval. Effect time — plasma concentration data were submitted to nonlinear regression analysis, which showed that the observed BP effects could be dissociated into depressor and pressor components. A window for the antihypertensive effect was established. At a plasma clonidine concentration of 0.65±0.07 ng/ml 50% of the maximal depressor effect was found, and it was only separated by a factor of 2 from the half maximal pure pressor concentration in plasma. No relationship between the change in heart rate and the plasma clonidine was observed. The findings strengthen the importance of close monitoring of clonidine therapy.SMR Project B81-04-054-14-04     

Pharmacological studies on the hypotensive effects of clonidine

The arterial pressure and heart rate of cats anaesthetized with urethane and chloralose were measured following treatment with clonidine (5 μg/kg). Blood pressure and heart rate were consistently decreased, effects which were blocked by haloperidol and phenoxybenzamine in doses which block central noradrenaline (NA) receptor sites; antagonism did not seem to be related to their peripheral adrenolytic properties. Pimozide and spiroperidol, in doses which block central dopamine but not NA receptor sites, did not block the hypotensive effects of clonidine. Pimocide and spiroperidol alone caused slight hypotension and bradycardia which, at least with regard to pimozide were not related to peripheral α-adrenergic receptor blocking activity. Clonidine potentiated these effects of pimocide and spiroperidol. The present findings indicate that the hypotensive effect of clonidine is due to stimulation of central NA receptor sites. Furthermore, dopaminergic mechanism may also be involved in vasomotor regulation.     

Endothelin antagonizes the hypotension and potentiates the hypertension induced by clonidine

Modification of clonidine-induced cardiovascular effects by endothelin-1 (ET-1) was studied in male Sprague-Dawley rats. A dose-dependent decrease in blood pressure and heart rate was produced by clonidine (100, 250 and 500 μg/kg i.v.). Lower doses produced only a fall in blood pressure (through central -adrenoceptors) while higher doses of clonidine produced an initial hypertensive response (through peripheral -adrenoceptors) and subsequent longer lasting hypotension and bradycardia. The hypotension and bradycardia induced by 100 and 250 μg/kg i.v. dose of clonidine were completely blocked by ET-1 (100 ng/kg i.v.) pretreatment. Conversely, the hypertensive response induced by high dose of clonidine (500 μg/kg i.v.) was significantly potentiated by ET-1 pretreatment. In cervical sectioned rats, i.v. administered clonidine failed to produce any hypotensive effect, indicating lack of central effect of clonidine. ET-1 significantly (P< 0.0005) potentiated the hypertensive response of a low dose (50μg/kg i.v.) of clonidine in cervical-sectioned rats. I.c.v administration of clodinine (1, 2, 4 and 6 μg) produced a dose-dependent decrease in blood pressure and heart rate. ET-1 pretreatment (25 ng i.c.v. transietly blocked the clonidine-induced decrease in blood pressure and heart rate for about 10 min but the hypotension and bradycardia was observed subsequently. Since the major site of action of clonidine is the ventral surface of medulla, clonidine was applied directly to the ventral surface of medulla and produced a decrease in blood pressure and heart rate. ET-1 pretreatment at the ventral surface of medulla blocked the clonidine-induced decrease in blood pressure and heart rate initially but the fall in blood pressure and heart rate was observed subsequently. To explore the possibility that transient antagonism of clonidine-induced effects is due to vasoconstriction, studies were performed with angiotensin II, a powerful vasoconstrictor. Angiotensin II (5 μg i.c.v.) pretreatment like ET-1 blocked the hypotensive and bradycardic effect of i.c.v. or i.v. administered clonidine. It is concluded that ET-1 blocks the hypotensive and potentiates the hypertensive effect of clonidine, possible mechanisms have been discussed.     

Sedation and plasma concentration of clonidine hydrochloride for pre-anesthetic medication in pediatric surgery

Clonidine hydrochloride has been used for pre-anesthetic medication to provide a pre-operative sedation in pediatric surgery. The purpose of this study is to determine the plasma clonidine concentration, which gives satisfactory sedation in pediatric surgery. Sixteen pediatric patients (age: 1-11 years, weight: 9-33 kg) received either 2 or 4 microg/kg of clonidine lollipop before entering the operating room. Plasma clonidine concentrations were determined 120 min after administration of clonidine lollipop. Pre-operative sedation was evaluated by 5-point scoring systems at entering the operating room. The changes in systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (HR) were also assessed before and after administration of clonidine lollipop. The patients with satisfactory sedation had higher plasma clonidine concentration than that of the patients with unsatisfactory sedation (0.45+/-0.16 ng/ml vs. 0.26+/-0.16 ng/ml, p<0.05). The clonidine concentrations in the satisfactory group ranged from 0.28 to 0.81 ng/ml. There was no significant difference in hemodynamic parameters (SBP, DBP and HR) before and after administration of clonidine lollipop in both satisfactory and unsatisfactory sedation groups. Plasma clonidine concentration of 0.3-0.8 ng/ml would be sufficient to produce satisfactory sedation without changes in hemodynamic parameters in pediatric surgery.     

Kinetics of ajmaline disposition and pharmacologic response in beagle dogs

Pharmacokinetics and pharmacodynamics of ajmaline were studied in four healthy dogs after intravenous administration of the drug at the infusion rate of 1.0 mg/min for 45 min. Ajmaline exhibited a saturable binding to plasma protein. One kind of binding site was found in the range of observed drug concentrations and its binding capacity showed nearly threefold interindividual difference. The time course of ajmaline concentration in whole blood Cb could be described by the two-compartment open model and the unbound concentration of ajmaline in plasma Pf was estimated from Cb by using the hematocrit value and the parameters of plasma protein binding and erythrocyte partitioning. The pharmacologic responses to ajmaline were assessed by recording ECG, and the changes in PQ and QRS interval were studied in relation to ajmaline disposition. When ECG changes were related to the ajmaline concentration, a significant degree of hysteresis was observed. The relationship between the unbound drug concentration and the pharmacologic effect was analyzed by a combined pharmacokinetic-pharmacodynamic model, where the hypothetical effect compartment is connected to the Pf in the central compartment by a first-order process. This model allows estimation of the changes in PQ and QRS intervals after intravenous administration of ajmaline. By comparing the drug effect on PQ and QRS intervals, it was suggested that ajmaline distributes to the atrial and the ventricular tissue in a similar degree and causes a reduction in the conduction rate in both sites with similar activity.     

Evidence for the involvement of central I1 imidazoline receptor in ethanol counteraction of clonidine hypotension in spontaneously hypertensive rats

Our previous studies have shown that ethanol counteracts centrally mediated hypotensive responses to clonidine. In this study, we investigated the relative roles of central alpha2-adrenergic and I1 imidazoline receptors in the antagonistic ethanol-clonidine hemodynamic interaction. The effects of selective blockade of alpha2- or I1 receptor by 2-methoxyidazoxan and efaroxan, respectively, on the blood pressure and heart rate responses to clonidine and subsequent ethanol administration were evaluated in conscious spontaneously hypertensive rats. Intracisternal administration of clonidine (1.5 microg/kg) produced significant (30 mm Hg; p < 0.05) and sustained (at least 60 min) decreases in blood pressure and heart rate. Systemic ethanol (1 g/kg), administered 10 min after clonidine, counteracted the hypotensive response and restored blood pressure to the preclonidine levels. Treatment with 2-methoxyidazoxan (0.16 microg/kg, intracisternal) or efaroxan (0.45 microg/kg, intracisternal) produced similar attenuation of the hypotensive and bradycardic responses to clonidine. The ability of ethanol to counteract the hypotensive action of clonidine was significantly (p < 0.05) attenuated in rats pretreated with efaroxan. The pressor response to ethanol lasted only 10 min compared with at least 60 min in the absence of efaroxan. In contrast, ethanol counteraction of clonidine-evoked hypotension was not altered when alpha2-adrenoceptors were blocked by 2-methoxyidazoxan. These findings suggest that centrally mediated hypotensive and bradycardic effects of clonidine in conscious spontaneously hypertensive rats involve activation of both alpha2-adrenergic and I1 imidazoline receptors. Furthermore, the findings suggest the dependence of a fully expressed ethanol counteraction of the hypotensive action of clonidine on functional I1 receptor within the central nervous system.     

Studies on the anti-vasoconstrictor activity of BRL 34915 in spontaneously hypertensive rats; a comparison with nifedipine.

1. The blood pressure lowering and anti-vasoconstrictor effects of BRL 34915 and nifedipine were compared in female spontaneously hypertensive rats (SHR). 2. In conscious SHR, intravenous injection of BRL 34915 (0.1, 0.3 mg kg-1) produced rapid, dose-related falls in mean arterial pressure of greater than 3 h duration. Nifedipine, at the same intravenous dose levels, also evoked rapid anti-hypertensive effects, though these responses were of lesser magnitude and duration than those observed for BRL 34915. 3. In anaesthetized, ganglion-blocked SHR, BRL 34915 (0.1, 0.3 mg kg-1 i.v.) dose-dependently antagonized the pressor responses to incremental intravenous infusions of noradrenaline (3.8-28.5 ng min-1) or phenylephrine (120-907 ng min-1) but did not inhibit pressor responses to incremental infusions of methoxamine (0.47-3.63 micrograms min-1), angiotensin II (7.0-52.9 ng min-1) or vasopressin (0.27-2.0 mu min-1). 4. In anaesthetized, ganglion-blocked SHR, nifedipine (0.1, 0.3 mgkg-1 i.v.) antagonized the pressor responses to each of the infused vasoconstrictor agents, being most effective against responses to noradrenaline or angiotensin II. 5. In pithed SHR, both BRL 34915 and nifedipine (each at 0.3 mg kg-1 i.v.) reduced the basal blood pressure level and produced marked inhibition of frequency-dependent pressor responses evoked by electrical stimulation of the spinal cord sympathetic outflow (0.25-4.0 Hz). Restoration of the basal diastolic blood pressure to within the control range, using a continuous intravenous infusion of vasopressin (0.98 mu min-1), prevented the inhibitory effect of BRL 34915. In the case of nifedipine, however, even raising the basal blood pressure to a level exceeding that recorded in control rats (with vasopressin, 2.0 mu min-1), did not reverse the inhibitory effect of the drug on frequency-dependent pressor responses. 6. It is concluded that the anti-hypertensive properties of BRL 34915 in SHR are probably unrelated to an anti-vasoconstrictor action. In contrast, it is suggested that the broadly-based anti-vasoconstrictor properties of nifedipine may contribute substantially to the anti-hypertensive properties of this drug.     

The cardiovascular actions of clonidine and neuropeptide-Y in the ventrolateral medulla of the rat.

1. The cardiovascular responses to neuropeptide-Y (NPY) (25 and 50 pmol) and clonidine (10 and 20 nmol) were examined following microinjection into the rostral ventrolateral medulla (RVLM) and the caudal ventrolateral medulla (CVLM). Mean arterial pressure (MAP) and heart rate (HR) were measured in anaesthetized rats, pre- and post-injection. 2. The alpha 2-adrenoceptor agonist clonidine (10 and 20 nmol) reduced MAP and HR significantly when microinjected into the CVLM and RVLM. 3. NPY (25 and 50 pmol) microinjected into the CVLM decreased MAP and HR. However, in the RVLM neither dose had a significant cardiovascular effect. 4. The possibility of a functional interaction between the adrenergic system and NPY was examined by co-administration of clonidine and NPY in doses that gave submaximal blood pressure responses. In the CVLM this produced hypotension and bradycardia which was similar in magnitude to the sum of their individual responses, indicating that in this area their actions appear to be independent. 5. In the RVLM, where NPY has no significant cardiovascular effects, co-administration with clonidine, did not alter the response to clonidine. 6. It appears that in the areas investigated, there is no functional interaction between NPY and clonidine.     

Disposition and tissue distribution of angiopeptin in the rat.

The disposition and tissue distribution of angiopeptin, a long-acting octapeptide analogue of somatostatin, were studied in rats following single iv and sc administration of the drug. Similar plasma levels and excretion values of angiopeptin were observed by using radioimmunoassay and radiolabeling techniques. Angiopeptin was absorbed fairly rapidly, with a mean peak plasma level of 25 +/- 4.1 ng/ml at 10-15 min after administration. The kinetics of angiopeptin following sc administration closely resembled those following iv administration due to rapid absorption. The pharmacokinetics of angiopeptin can be described by a two-compartment model. The plasma half-life of the drug ranged from 2.6-2.9 hr when administered sc and 1.98-2.5 hr when given iv. Distribution of angiopeptin was rapid, with the highest concentration appearing in the liver. Half-lives in the liver and bile were short. Most of the drug was excreted in the feces via the bile, while approximately 10% was excreted in the urine. Angiopeptin was also found to be secreted in the saliva. TLC and HPLC of blood, urine, feces, and bile samples did not reveal the presence of any metabolites. In conclusion, the in vivo fate of angiopeptin is characterized by little or no hepatic metabolism and rapid biliary excretion.