INCREASE OF THE DELAYED RECTIFIER K+ AND NA+-K+ PUMP CURRENTS BY HYPOTONIC SOLUTIONS IN GUINEA-PIG CARDIAC MYOCYTES

To investigate the membrane current changes induced by membrane stretching, single guinea pig ventricular myocytes were superfused with solutions of various osmolarities, and the whole-cell current was recorded by the patch-clamp technique. The application of 70% and 130% osmolar bath solutions increased and decreased the amplitude of delayed rectifier K+ current (I-K), respectively, whereas no obvious change was observed in the L-type Ca2+ current or the inward rectifier K+ current. When the Na+-K+ pump current (I-pump) was recorded by the use of high-Na+ (>35 mmol/L) pipette solutions, I-pump was also increased and decreased by the superfusion of hypotonic and hypertonic solutions, respectively, in approximately half of the cells. An increase of the I-pump was also observed in the absence of external Na+, excluding a possibility that the enhancement of I-pump was secondary to an elevation of cytosolic Na+. In most cells that did not show the increase of I-pump, the hypotonic superfusion induced a gradual activation of Cl- current. The hypertonic superfusion did not cause any consistent change in the membrane Cl- conductance. Since the response of I-K was observed in all experiments, its mechanism was studied. We failed to observe marked changes in the kinetic and conductance properties of I-K in the hypotonic solution. The involvements of either the protein kinases or Ca2+ were also ruled out as major mechanisms underlying the I-K response.