Discrimination of A1 versus A2 receptor subtype selectivity of adenosine receptor agonists in vivo.

Previous attempts to discern and quantify the selectivity of agonists for A1 versus A2 adenosine receptors in vivo have been confounded by the activation of baroreceptor reflexes and/or simultaneous expression of responses to both A1 and A2 receptor activation. In anesthetized, vagotomized rats with isolated in situ constant-flow perfused hindquarters (HQ), bradycardic responses to i.v. agonist injections measured A1 receptor activation and HQ vasodilation elicited by i.a. agonist injections measured the stimulation of A2 receptors. Adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) produced A2 receptor-mediated HQ vasodilation at doses 8- and 4-fold lower (-log ED50 values, 7.3 +/- 0.04 mol and 8.7 +/- 0.06 mol, respectively) than those required to evoke A1 receptor-mediated bradycardia (-log ED50 values, 6.4 +/- 0.01 mol and 8.1 +/- 0.07 mol, respectively). N6-cyclopentyladenosine (CPA) was approximately 8-fold selective for A1 receptors (-log ED50 values, A1, 8.5 +/- 0.05 mol; A2, 7.6 +/- 0.16 mol). 2-(Phenylamino)adenosine (CV-1808) and 2[2(4-fluorophenyl)ethoxy]adenosine (FPEA) were at least 125- and 200-fold more potent agonists at A2 receptors (-log ED50 values, 7.7 +/- 0.10 mol and 8.0 +/- 0.24 mol, respectively) than at A1 receptors (-log ED50 values, 5.6 +/- 0.08 mol and 5.7 +/- 0.01 mol, respectively). These studies demonstrated that stimulation of A1 and A2 receptors may be discriminated in vivo and that such responses are selective, reproducible, dose-dependent and quantifiable.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of adenosine A1 receptor-mediated renal vasoconstriction with endogenous nitric oxide and ANG II.

Renal vasoconstrictor responses to the adenosine A1 agonist N6-cyclopentyladenosine (CPA) were compared in the in situ autoperfused rat kidney to responses evoked by angiotensin II (ANG II), endothelin-1 (ET-1), arginine vasopressin (AVP), carbocyclic thromboxane A2 (CTxA2), phenylephrine (PE), and 5-hydroxytryptamine (5-HT). On the basis of their ED50 values (dose of agonist, in mass units, that produced 50% of maximal response to that agonist), the order of vasoconstrictor potency was ANG II > or = AVP > ET-1 > CPA > 5-HT > or = PE > CTxA2. Dose-response curves to CPA were shallower and maximal responses were weaker than those produced by the other agonists. Maximal responses, the log ED50, and the slope of the dose-response curve to CPA were markedly potentiated in the presence of the nitric oxide (NO) synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME). Selective antagonism of A1 receptors increased renal blood flow and markedly attenuated CPA-induced renal vasoconstriction in the absence or presence of L-NAME but had no effect on the maximal responses to ANG II. Conversely, AT1 receptor antagonism attenuated renal vasoconstriction produced by ANG II but had little effect on the produced by CPA. These results suggest that endogenous NO modulates renal vasoconstriction produced by A1 receptor stimulation and provide evidence against an interaction between renovascular adenosine A1 and angiotensin AT1 receptors.

N-0861 selectively antagonizes adenosine A1 receptors in vivo.

Experiments were performed to determine the antagonistic actions of (+/-)N6-endonorbornan-2-yl-9-methyladenine (N-0861) at A1 and A2 adenosine receptors in vivo, and to evaluate the pharmacodynamics of the observed responses. The selectivity of antagonism of A1 vs. A2 receptors by N-0861 was evaluated by generating dose-response curves to adenosine-induced bradycardia (A1 effect), and vasodilation in the in situ constant-flow perfused rat hindquarter vasculature (A2 effect). N-0861, at doses greater than or equal to 1 mumol/kg + 0.04 mumol/kg per min, i.v. produced dose-related rightward shifts of the A1 dose-response curve, but had no effect on the A2 dose-response curve at doses as high as 100 mumol/kg, i.v. In contrast, the non-selective A1/A2 adenosine receptor antagonist 8-phenyltheophylline antagonized both A1 and A2 receptor-mediated responses to adenosine. The minimum effective i.v. dose and the duration of action of N-0861 were determined by evoking bradycardic responses to i.v. adenosine (A1 effect) in anesthetized, vagotomized, beta-blocked rats before and after single bolus doses of vehicle or N-0861 (0.3, 0.6, 1.0, 3.0 or 10.0 mumol/kg). The lowest i.v. dose of N-0861 to antagonize A1 receptor-mediated bradycardia was 0.3 mumol/kg i.v.; the duration of effect ranged from 1 min (following 0.3-1 mumol/kg) to approximately 2.5 h (following 10 mumol/kg). N-0861 is a selective (by greater than or equal to 333-fold) antagonist of adenosine A1 receptors.

A simplified method for assessing drug effects on gastric emptying in rats.

A method is described to determine the effects of experimental compounds on gastric emptying in rats. The technique, using a methylcellulose-based test meal, is simple, versatile and reliable. Data are presented to show the utility of the method in demonstrating both enhancement and inhibition of gastric emptying.

Effect of an inhibitor of pepsin proteolytic activity in the Exalto-Mann-Williamson ulcer.

The role of pepsin proteolytic activity in the Exalto-Mann-Williamson ulcer has previously not been established. Our studies show that a potent inhibitor of pepsin activity, when administered orally to E-M-W dogs, reduced both the incidence and severity of jejunal ulceration. These results indicate that pepsin may play a major role in the development of ulceration in this model.