INVOLVEMENT OF INTRACELLULAR CA-2-INDUCED CARDIAC CA-2+ CURRENT INHIBITION( RELEASE MECHANISM IN ADENOSINE)

Background. Myocardial contractility is regulated by changes in the fr ee intracellular Ca2+ concentration Ca2+!i, which is determined by co ncerted interactions of Ca2+ influx through voltage-dependent Ca2+ cha nnels, release of Ca2+ from intracellular pools, and Ca2+ extrusion sy stems. Although adenosine has been reported to attenuate postischemic stunning, the mechanism is unknown. Methods. The patch clamp technique and the Ca2+-sensitive fluorescence dye FURA II were used in isolated ventricular cardiomyocytes to study the effects of adenosine on the L -type Ca2+ channel current (I(Ca)) and on Ca2+!i. Results. Adenosine decreased the basal I(Ca) by 76% +/- 4% (100 nmol/L, n = 48) and simul taneously elevated Ca2+!, Up to 1.8 mumol/L (n = 37) by adenosine-A1- receptor activation. The adenosine-evoked responses were not sensitive to pertussis toxin (n = 12), but intracellular application of guanosi ne diphosphate-beta sulfate (10 mumol/L) reduced the effects of adenos ine by 86.7% +/- 7% (n = 9). Block of intracellular Ca2+ release by ry anodine and thapsigargin attenuated, whereas intracellularly applied i nositol 1,4,5-trisphosphate (n = 18) mimicked the adenosine-evoked res ponses. Conclusions. These findings suggest that coupling of adenosine to A1-receptors induced G protein-mediated activation of phospholipas e C followed by generation of inositol 1,4,5-trisphosphate, which rele ases Ca2+ from intracellular Ca2+ stores. The inhibition of I(Ca) by d epletion of intracellular Ca2+ stores could play a major role in preve nting ischemia-induced Ca2+ overload.