EFFECT OF HYPEROSMOTIC SOLUTIONS ON INTRA CELLULAR FREE CA2-CELLS( CONCENTRATION IN SINGLE ENZYMATICALLY ISOLATED CARDIAC)

Intracellular free Ca2+ ([Ca2+]i) was measured in single isolated rat ventricular myocytes using the fluorescent probe Fura 2/AM. The restin g (basal) level of [Ca2+]i in single cardiomyocytes averaged 50 +/- 8 nM (n = 48). Replacement of the control HEPES-Ringer solution (CHRS) w ith hyperosmotic solution (HSS) made by adding 585 mM sucrose to CHRS induced a rapid increase in [Ca2+]i by 50-100 nM (above the basal leve l), followed by its slow increase. Removal of Ca2+ from extracellular solution prior to applying HSS led to a drastic decrease in the [Ca2+] i response to HSS. Depletion of intracellular Ca2+ stores by caffeine (10 mM) abolished completely the initial rapid rise in [Ca2+]i induced with HSS. Thus, [Ca2+]i response to HSS is due to both Ca2+ release f rom intracellular stores (the initial peak) and Ca2+ influx from extra cellular medium (slow [Ca2+]i increase).