An unusual receptor mediates adenosine-induced SA nodal bradycardia in dogs

Abstract

To characterize the receptor mediating the negative chronotropic effect of adenosine in dogs, experiments were performed on conscious dogs with chronically implanted cardiovascular instrumentation. Autonomic blockade was used to eliminate any reflex influences on heart rate. Intravenous bolus injections of various adenosine analogues caused dose-dependent, aminophylline-blockable reductions in heart rate with a potency order of 5'-(N-ethylcarboxyamido)-adenosine (NECA)-78:2-chloroadenosine-17:adenosine-1. Dipyridamole enhanced the potency of adenosine to equal that of 2-chloroadenosine. Moderately selective A1-receptor agonists N6-(L-2-phenylisopropyl)-adenosine (R-PIA) and N6-cyclohexyladenosine and an A2-selective agonist 2-phenylaminoadenosine (200 nmol/kg) had no negative chronotropic effect in the conscious dog. Adenosine and its analogues, including R-PIA, caused coronary vasodilatation at smaller doses than were required to slow the heart rate. The selective A1-adenosine receptor blocker xanthine amine congener (XAC) antagonized the negative chronotropic action of adenosine but did so nonselectively, as the coronary vasodilative and negative chronotropic actions of adenosine were antagonized equally well. The spontaneous contraction rate of isolated perfused dog right atrial preparations, which included the sinoatrial node, was reduced by intrasinoatrial node artery infusions of adenosine analogues with a potency ratio of NECA-100:adenosine-15:N6-cyclopentyladenosine-2.3:R-PIA-1. We conclude that the adenosine receptor mediating the negative chronotropic action of adenosine in the dog does not display the pharmacological characteristics of either typical A1- or A2-adenosine receptors. Instead, either a novel adenosine receptor or an A1-receptor with unusual agonist and antagonist binding properties appears to exist in the dog's sinoatrial node.

Comparison of Three Different A1Adenosine Receptor Antagonists on Infarct Size and Multiple Cycle Ischemic Preconditioning in Anesthetized Dogs

“…Our results demonstrate that A 1 AR blockade is not detrimental in this experimental model and does not block IPC induced by multiple ischemia/reperfusion cycles. Although caution is certainly necessary to extrapolate our findings in a dog model to humans, especially since it has been suggested that A 1 ARmediated responses in the dog may differ from that of humans (Belloni et al, 1989;Martin, 1992), our data provide evidence that the use of A 1 AR antagonists as diuretics inpatients with ischemic heart disease may not pose a problem. Dose-relationship studies in additional models will be necessary to address this issue further.…”

Inhibition of lipolysis causes suppression of endogenous glucose production independent of changes in insulin

“…This was not changed by Liposyn or glycerol control infusion and therefore was not likely due to the effect of CHA to lower FFA or glycerol. It is possible E635 that CHA had some direct effect on peripheral tissues to increase glucose uptake, perhaps by increasing regional blood flow (3).…”

Cardiovascular selectivity of adenosine receptor agonists in anaesthetized dogs

“…Indeed, some reports have indicated that Ado receptors involved in canine sino-atrial (SA) nodal function in vivo '." Macmillan Press Ltd, 1992 have an unusual order of agonist potency that does not conform to the conventional Al receptor (Belloni et al, 1989a) identified from tissue and binding studies in vitro.…”