The role of alpha 1-adrenoceptor subtypes in the regulation of arterial blood pressure.

The effect of chlorethylclonidine on alpha 1-adrenoceptor subtypes and arterial blood pressure has been evaluated. Chlorethylclonidine significantly reduced the alpha 1-adrenoceptor population. Chlorethylclonidine treatment had no significant effect on resting systemic arterial blood pressure or heart rate and shifted the phenylephrine pressor dose-response curve only 2.4-fold to the right. These data suggest that only one of the alpha 1-adrenoceptor subtypes plays a major role in the regulation of arterial blood pressure.

Evidence for a complex interaction between the subtypes of the alpha 1-adrenoceptor.

Ligand binding studies with WB 4101 revealed that the rat aorta contains both the alpha 1a- and alpha 1b-adrenoceptor subtypes. Results obtained following treatment with the irreversible antagonists phenoxybenzamine, chlorethylclonidine or SZL-49 (4-amino-6,7-dimethoxy-2-quinazolinyl-4-(2-bicyclo[2,2,2]octa-2,5- dienylcarbonyl-2-piperazine) suggest that there is a complex interaction between the alpha 1-adrenoceptor subtypes in the aorta. Chlorethylclonidine affects only the alpha 1b-adrenoceptor, whereas the predominant action of SZL-49 is on the alpha 1a-subtype. Chlorethylclonidine significantly inhibited the response to either methoxamine or phenylephrine, agents which are selective alpha 1a-adrenoceptor agonists. Following inactivation with either chlorethylclonidine or SZL-49, the response of the rat aorta to phenylephrine was only partially antagonized by either prazosin or WB 4101. SZL-49 also inhibited the response of the rat tail artery to electrical stimulation. The response of the tail artery obtained following inactivation with SZL-49 was effectively antagonized by prazosin. Phenylephrine, prazosin or WB 4101 afforded complete protection from chlorethylclonidine adrenoceptor inactivation, while these same ligands were only partially effective against SZL-49. Either SZL-49 or chlorethylclonidine significantly impaired the irreversible adrenoceptor blocking actions of phenoxybenzamine. These results suggest: (1) only the alpha 1a-adrenoceptor subtype appears to be associated with nerve terminals in the tail artery, (2) there may be a complex interaction between the alpha 1-adrenoceptor subtypes such that both receptors must be intact and functional to observe normal agonist and antagonist interactions, (3) there may be three sites of action for agonists associated with the rat aorta.

The effects of flumazenil-precipitated abstinence on the pharmacokinetics of chronic oxazepam in dogs.

The pharmacokinetics of oxazepam was studied in naive dogs and in oxazepam-dependent dogs without and with administered flumazenil (6 mg/kg). Oxazepam is eliminated with a relatively short elimination half life (ca. 150 min) in both acutely and chronically treated dogs. It exhibits only a modest first pass metabolism (ca. 10%) and its bioavailability following oral administration is about 22%. The steady state concentration of oxazepam in chronically treated dogs was lower than was predicted from single dose studies. Flumazenil did not change the rate of absorption or elimination of oxazepam-dependent dogs. The total steady state plasma concentration of oxazepam was significantly reduced by flumazenil administration suggesting a displacement interaction between flumazenil and oxazepam.

Interaction of imidazolines with alkylation-sensitive and -resistant alpha-1 adrenoceptor subtypes.

The interaction of imidazolines with alpha-1 adrenoceptor subtypes sensitive and resistant to inactivation by SZL-49 and chlorethylclonidine (CEC) has been evaluated. Clonidine, oxymetazoline, phentolamine and naphazoline or the phenethylamine, phenylephrine, interacted with high- and low-affinity sites labeled by [3H]prazosin. SZL-49 (1-1000 nM) eliminated the high-affinity sites and caused a significant reduction of the low-affinity sites. CEC (1-100 microM) reduced the number of low-affinity sites, while the effect on high-affinity sites was dependent on the route of administration. In control aortic rings the dose-response curves for either clonidine or naphazoline were biphasic, consisting of high- and low-affinity components. Only the high-affinity component was blocked by prazosin. SZL-49 was more potent than CEC at inhibiting agonist-induced contraction of rat aortic rings. The agonist responses obtained after treatment with either SZL-49 or CEC were only weakly antagonized by prazosin. The combination of SZL-49 and CEC produced no greater inhibition of muscle contraction than did SZL-49 alone. These data show that 1) imidazolines interact with different affinity at sites labeled by [3H]prazosin and these sites correspond to the alpha-1a and alpha-1b adrenoceptor subtype designation; 2) imidazolines induce smooth muscle contraction by interacting at high- and low-affinity sites; 3) these low-affinity sites do not appear to have properties of an alpha-1 adrenoceptor; 4) there may be three sites of interaction for imidazolines on the aorta, the alpha-1a and alpha-1b adrenoceptors and a site that does not have alpha-1 adrenoceptor characteristics.

Effect of chlorethylclonidine on arterial blood pressure and heart rate in the conscious rat.

The effect of chlorethylclonidine (CEC) on arterial blood pressure and heart rate (HR) has been evaluated in the conscious rat. CEC injection (25 mg/kg i.p.) caused a statistically significant decrease in mean arterial blood pressure (MAP) that was seen 24 hr after treatment. CEC also induced a decrease in HR that was maximal at 45 min but returned to pretreatment levels after 3 hr. CEC had no effect on the ability of isoproterenol to increase HR. CEC treatment had little effect on the pressor dose-response curve of either phenylephrine or BHT 920. When injected into the brain (25 mg/kg, lateral ventricle), CEC had no effect on MAP or HR. Yohimbine injected into the lateral ventricle had no effect on the response to i.p. CEC. Prazosin, used as a standard for comparison, caused a larger fall in MAP than CEC and this hypotension was associated with tachycardia and a marked shift (greater than 300-fold) in the phenylephrine pressor dose-response curve. A reactive analog of prazosin, SZL-49 [1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(2-bicyclo[2,2,2]octa- 2,5-diene-2-carbonyl)piperazine], had effects similar to prazosin on MAP and HR.(ABSTRACT TRUNCATED AT 250 WORDS)

N-desmethyldiazepam physical dependence in dogs.

Dogs, surgically implanted with a chronic gastric fistula, were chronically dosed with N-desmethyldiazepam (32 mg/kg/day) in four divided doses to attain N-desmethyldiazepam plasma levels comparable to those observed in dogs dependent on diazepam (60 mg/kg/day). The time course of N-desmethyldiazepam abstinence was studied, beginning not less than 2 weeks after stabilization levels had been achieved. The abstinence syndrome observed after abrupt discontinuation of N-desmethyldiazepam was similar to the diazepam abstinence syndrome but differed in several important aspects. In diazepam-dependent dogs, there was a short burst of tremor very early in withdrawal (approximately 1-2 hr after the last dose of diazepam) that was not seen in N-desmethyldiazepam-dependent dogs. Signs of abstinence such as tremor, hot foot walking and twitches and jerks were more frequently observed in N-desmethyldiazepam-dependent dogs than in diazepam-dependent dogs as were decreases in food and water intake and in body weight. The overall intensity of abstinence, as measured by the Diazepam Withdrawal Abstinence Scale, was greater in N-desmethyldiazepam-dependent dogs than in dogs dependent on either lorazepam or diazepam. Plasma levels of N-desmethyldiazepam and oxazepam were nearly equal in dogs dependent on diazepam or on N-desmethyldiazepam and were 4 to 10 times greater than the plasma levels of diazepam or lorazepam in diazepam- or lorazepam-dependent dogs, respectively. Furthermore, the plasma levels of N-desmethyldiazepam and oxazepam declined much more slowly than the levels of diazepam and lorazepam. These results suggest that physical dependence on diazepam is caused by the accumulation and actions of N-desmethyldiazepam.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist interaction with alkylation-sensitive and -resistant alpha-1 adrenoceptor subtypes.

The interaction of agonists with alpha-1 receptor subtypes sensitive and resistant to alkylation by a prazosin analog [1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(2-bicyclo[2.2.2]octa-2,5- diene-z-carbonyl)-piperazine; SZL-49] has been examined. In rat aortic rings, SZL-49 (0.1-10 nM) shifted the dose-response curves for norepinephrine and phenylephrine to the right. The curves were biphasic, consisting of high and low affinity components. At greater than 10 nM, the curves became monophasic. After SZL-49 treatment, the response to norepinephrine was partially antagonized by diltiazem. Chlorethylclonidine (1-100 microM) also produced biphasic dose-response curves. Phenylephrine bound to high and low affinity sites labeled by [3H]prazosin, and the high affinity site was eliminated by SZL-49. SZL-49 (i.p.) shifted the pressor dose-response curve for phenylephrine to the right but did not decrease the maximal response. Chlorethylclonidine was much less potent than SZL-49 at shifting the pressor dose-response curve. Pertussis toxin, 50 micrograms/kg i.v., shifted the phenylephrine pressor dose-response curve in control and SZL-49-treated animals. SZL-49 inhibited norepinephrine-induced inositol phosphate formation, whereas chlorethylclonidine had no effect on inositol phosphate formation. These data show: 1) both in vitro and in vivo, alpha-1 receptor subtypes sensitive and resistant to alkylation by SZL-49 can mediate the full response of agonists; 2) these subtypes exhibit high and low affinity for agonists; 3) responses mediated by either subtype are partially dependent on calcium channel activity and a pertussis toxin-sensitive G-protein; 4) the SZL-49 sensitive site is able to enhance the formation of inositol phosphates.