Factors affecting the contribution of the catecholamine activated electrogenic sodium pump to the membrane potential of rat soleus muscle fibers

Abstract

1. 1. The effects of several drugs and ionic environments on the ability of catecholamines to affect the membrane potential of rat soleus muscle fibers were studied in vitro. 2. 2. Maximum hyperpolarization is achieved with 10 −5 M noradrenaline or 10 −6 M adrenaline and is blocked by propranolol (10 −6 M) but not phentolamine (10 −5 M). 3. 3. The noradrenaline hyperpolarization is sensitive to temperature and decreases by 39% between 31°C and 22°C and disappears by 14°C. 4. 4. The hyperpolarization is maximal (10.1 mV) when the external potassium concentration is near normal plasma levels, but decreases at lower potassium concentrations and disappears at potassium concentrations of 10.8 mM or above. 5. 5. The hyperpolarization is not due to a decrease in sodium permeability because in Kreb's solutions when sodium is partially or totally replaced by lithium, noradrenaline causes a depolarization. 6. 6. At those potassium concentrations where noradrenaline is able to hyperpolarize the muscle, ouabain (10 −4 M) blocks this hyperpolarization, depolarizes the fibers and causes fibrillations. A ouabain-induced depolarization is observed in Kreb's solutions where the sodium is partially replaced by lithium or choline. Fibrillations are not observed when ouabain is added to these solutions. 7. 7. Removal of calcium from or addition of 1 mM EGTA to normal Kreb's solution decreases, but does not block the noradrenaline hyperpolarization, while removal of calcium and inclusion of EGTA does block the hyperpolarization. The use of verapamil or D-600 does not affect the hyperpolarization in normal Kreb's, but does significantly reduce the hyperpolarization in calcium free Kreb's. 8. 8. Nicotine reduced and Nembutal had no effect on the hyperpolarization.