Ionic basis of the hyperpolarizing action of adenyl compounds on sinus venosus of the tortoise heart

Abstract

The ionic mechanism underlying the hyperpolarizing action of adenosine and adenine nucleotides was studied by measuring the efflux of 43K or 86Rb from sinus venosus and interauricular septum of tortoise heart. Preparations rendered quiescent by high-K (27 mM) Ringer solution were used. Adenosine and ATP increased the efflux of 43K and 86Rb from sinus venosus. The magnitude of the responses varied from preparation to preparation, but in the same tissue adenosine and ATP were of equal efficacy. When dose-response relationships could be determined, the adenyl compounds were found to be of similar potency. Km for adenosine was 6.2 X 10(-6) M, for ATP 8.3 X 10(-6) M. Regional variations in the magnitude of the responses were observed. The largest responses were obtained from the muscular strip of sinus venosus near its junction with atrium, and from the right horn of the sinus venosus. In interauricular septum the adenyl compounds caused only a slight increase in isotope efflux. Acetylcholine, by contrast, produced large increases in 86Rb efflux from all these preparations. Thus the distribution of the purinoceptors in the tortoise heart is more confined than that of the muscarinic receptors. Antagonism of the response to adenyl compounds by theophylline and 8-phenyltheophylline was studied. The apparent Ki for theophylline was 10(-5) M; that for 8-phenyltheophylline about 10(-6) M. Atropine did not inhibit the responses to the adenyl compounds. These results indicate that the changes in K permeability produced by adenosine and ATP are mediated by P1-purinoceptors. The adenosine transport inhibitors, dipyridamole and nitrobenzylthioinosine (NBMPR), had no effect on the adenyl-induced responses, indicating that adenosine uptake is of little importance in tortoise sinus venosus. The effects of phosphate-modified ATP analogues were studied. Adenylimidodiphosphate (APPNP) produced increases in 86Rb efflux similar to those found with ATP, confirming that breakdown of ATP to adenosine is not obligatory for its action at P1-purinoceptors. Alpha-beta methylene ATP (APCPP) and beta-gamma methylene ATP (APPCP) produced much smaller effects, which may be explained by their structural and chemical differences from ATP. The use of 86Rb as a tracer (Rb: K less than 0.01 in load solution) gives qualitatively similar results to those obtained when 43K is used to study the permeability increases produced by the adenyl compounds or acetylcholine. Quantitative differences in the measures obtained with the two isotopes, however, become apparent when the efflux of both is studied simultaneously.(ABSTRACT TRUNCATED AT 400 WORDS)