Harmaline distribution in single muscle fibres and the inhibition of sodium efflux

Abstract

Harmaline, a known inhibitor of the (Na+ + K+)-ATPase in cell membranes, inhibited 50% of the 22Na efflux from barnacle muscle fibres at an extracellular concentration of 2.4 mM. Injected harmaline inhibited 50% of the efflux at an estimated intracellular concentration of about 8 mM · kg−1, assuming complete equilibration with no binding. Total fibre harmaline was measured in separate fibres by ultraviolet spectrophotometry. Fibres in 3 mM harmaline saline accumulated harmaline with a half-time of 17 min and a final total fibre concentration of 6–12 mM · kg−1. In harmaline-free saline this accumulated harmaline was lost exponentially with a half-time of 35 min; injected harmaline was lost exponentially from fibres with a half-time of 50 min. It is proposed that harmaline crosses the fibre membrane as the uncharged base and that its apparent accumulation against a concentration gradient is mainly due to intracellular binding with an additional contribution from a transmembrane pH gradient. It is concluded that, in fibres exposed to harmaline saline, the intracellular concentration can reach a sufficiently high value, as judged from the results of the injection experiments, to inhibit Na+ efflux at an interior-facing site on the fibre membrane. In contrast, harmaline appears to inhibit the Na+-dependent uptake of l-glutamate at an extracellular site.