The electrogenic Na-pump and spontaneous contraction of the hypokalemic rat duodenum

The effects of the electrogenic Na-pump on spontaneous contraction in the isolated, longitudinal muscle of the duodenum of rats which had been on a potassium-deficient diet for 7 weeks, have been investigated. Intracellular levels of Na+ are increased by this diet. The spontaneous contraction of the duodenal muscle was stopped, transiently, by 0.5 to 120 mM-K+ Krebs solution. The period of decrease of tone and amplitude occurring immediately after adding K+ was shortened when the external K+ concentration ([K]o) was increased from 0.5 to 120 mM. The decrease in tone and amplitude induced by K+ was abolished by exposure of the tissue to 0 mM [K]o, by exposure to a temperature below 14 degrees C, and in the presence of ouabain (3 X 10(-5)-10(-4) M). The spontaneous contraction of 'Na-rich' duodenum in bathing medium containing 15 mM K+ and following inhibition of the electrogenic Na-pump with cooling or ouabain was much the same as in the duodenum from rats fed balanced diets: i.e., increase of contractile tone immediately after adding K+. To activate the Na-pump in 'Na-rich' duodenum, the external K+ could be replaced by Rb+, Cs+, NH4+ and Tl3+. The effectiveness was in the order K+ greater than Rb+ greater than Cs+ greater than NH4+ greater than Tl3+. The possible existence of a neuronal or hormonal inhibitory mechanism affecting the active Na-K transport in rat smooth muscle in situ, under conditions of hypokalemia, is discussed.