Role of ion conductance changes and of the sodium-pump in adrenaline-induced hyperpolarization of rat diaphragm muscle fibres.

The ionic mechanism of membrane hyperpolarization induced by adrenaline in rat diaphragm muscle fibres was studied. Removal of the extracellular K+ ([K+]o) from Krebs-Ringer solution initially increased the resting membrane potential and then caused an increase in the intracellular Na+ activity ([Na+]i) and a decrease in the intracellular K+ activity ([K+]i). All the changes were maintained for more than 3 h. Application of ouabain (0.1 mM) or lowering the temperature rapidly reduced the resting potential by about 10 mV in the K+-free solution. It then produced further progressive decreases in resting potential and in [K+]i and a progressive increase in [Na+]i. These observations indicate that an electrogenic Na-pump operates in the K+-free solution. Removal of most of the Cl- in the K+-free solution did not affect the resting potential or the magnitude of the initial decrease produced by ouabain, despite an increased input resistance; this result implies a passive distribution of Cl-. Adrenaline (30-60 microM) either added to the bathing solution or applied to the membrane by ionophoresis produced a hyperpolarization (3-10 mV: adrenaline hyperpolarization), the amplitude of which was decreased with a rise in [K+]o and increased with a reduction in [K+]o, but unaffected by the removal of Cl-. Adrenaline produced an increase in input resistance, the relative magnitude (17-18%) of which was constant whether external K+ or Cl- was removed. In contrast, a conditioning membrane hyperpolarization hardly affected the resistance. Ouabain (0.1 mM) or low temperature (8-10 degrees C) abolished both the hyperpolarization and the increased input resistance induced by adrenaline. The [K+]i, [Na+]i and the peak of the action potential remained unchanged after a 20 min exposure to adrenaline (30 microM). The hyperpolarization induced by the replacement of all Na+ with Tris (Tris-hyperpolarization) in the K+-free solution was depressed by 39% during the early period (4-31 min) of exposure to adrenaline (30 microM), while it was enhanced by 26% during the later period (80-130 min). The initial depression suggested a decrease in the ratio of the membrane permeability for Na+ (PNa) to that for K+ (PK). These results suggest that the adrenaline hyperpolarization is generated largely by a decrease in PNa/PK, which is associated with the activity of the Na-pump.