MEMBRANE CURRENTS UNDERLYING THE MODIFIED ELECTRICAL-ACTIVITY OF GUINEA-PIG VENTRICULAR MYOCYTES EXPOSED TO HYPEROSMOTIC SOLUTION

1. Guinea-pig ventricular myocytes were superfused with hyperosmotic ( sucrose) Tyrode solution (1.2-2.8 times (T) normal osmolality) for up to 40 min, Action potentials were recorded with microelectrodes, and m embrane currents with the perforated-or ruptured-patch technique. 2. H yperosmotic treatment for 20 min shrunk cell volume and hyperpolarized the membrane. Moderate (1.2-1.5T) treatment caused biphasic changes i n action potential configuration (rapid minor shortening quickly follo wed by lengthening to a stable 110% control duration). Severe (2.2-2.8 T) treatment caused triphasic changes (marked early shortening, strong rebound lengthening and subsequent pronounced shortening). At peak le ngthening (6-10 min) action potentials (165% control duration) had a h ump near -30 mV and slowed terminal repolarization. 3. In accordance w ith previous studies, hyperosmotic solution inhibited the delayed rect ifier K+ current, and enhanced the outward Na+-Ca2+ exchange current ( I-NaCa) at plateau potentials. A novel finding was that hyperosmolalit y reduced the amplitude of L-type Ca2+ current (I-Ca,I-L) and slowed i ts rate of inactivation. Experiments on myocytes loaded with indo-1 su ggest that the reduction in I-Ca,I-L is due to a rapid elevation of [C a2+](i). 4. When impaled myocytes were preloaded with EGTA, severe hyp erosmotic treatment induced a rapid monotonic shortening of the action potential to a stable 20% of control duration. Addition of external K + quickly nulled the hyperpolarization and slowly lengthened the actio n potential. 5. The results suggest that modified electrical activity in osmotically shrunken myocytes is primarily caused by increases in [ K+](i), [Na+](i) and [Ca2+](i): (i) elevated [K+](i) hyperpolarizes th e membrane (which may contribute to increased [Na+(])(i)); (ii) elevat ed. [Na+](i) shortens all phases of the action potential (increased ou tward-directed I-NaCs); and (iii) elevated [Ca2+](i) has antagonistic plateau shortening (inhibition of inward I-Ca,I-L) and plateau lengthe ning (reduced outward I-NaCa) influences, as well as a strong subplate au lengthening effect (enhanced inward I-NaCa).