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 patchclamp technique. The application of 70% and 130% osmolar bath solutions increased and decreased the amplitude of delayed rectifier K' current (IK), 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 (IpUmp) was recorded by the use of high-Na+ (>35 mmol/L) pipette solutions, '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 IpUmp was also observed in the absence of external Na+, excluding a possibility that the enhancement of IpUmp was secondary to an elevation of cytosolic Na+. In most cells that did not show the increase of 1pUmp. the hypotonic superfusion induced a gradual activation of Clcurrent. The hypertonic superfusion did not cause any consistent change in the membrane Clconductance. Since the response of 'K was observed in all experiments, its mechanism was studied. We failed to observe marked changes in the kinetic and conductance properties of 'K in the hypotonic solution. The involvements of either the protein kinases or Ca2' were also ruled out as major mechanisms underlying the IK response. (Circ Res. 1994;75: 887-895.)