On the temperature dependence of the Na pump in sheep Purkinje fibres

Abstract

The temperature dependence of cardiac active Na transport is studied in voltage clamped sheep Purkinje fibres by means of simultaneous measurements of the membrane current (I) and the intracellular Na activity (alpha iNa). During activation of the Na pump a transient outward current (delta I) and alpha iNa decline exponentially with an identical time constant (tau). The transient outward current and the decline in alpha iNa are blocked by 10(-4) M dihydroouabain (DHO). Lowering the temperature from 42 degrees C to 17 degrees C prolongs tau. The electrogenic fraction (e.f.) of the active Na efflux remains unaffected. The Q10 value of the active Na transport derived from the changes of tau varies within the temperature range studied. The Q10 amounts to approximately 1.2 between 42 degrees C and 35 degrees C, to approximately 2.4 between 35 degrees C and 22 degrees C and to approximately 2.1 between 35 degrees C and 17 degrees C. Correspondingly the activation energy of the active Na transport is not constant between 42 degrees C and 17 degrees C. It is calculated to be 3.4 kcal/mol between 42 degrees C and 35 degrees C, 15.9 kcal/mol between 35 degrees C and 22 degrees C and 12.4 kcal/mol between 35 degrees C and 17 degrees C. Variations in temperature change the maximal rate constant of the active Na transport, whereas the sensitivity of the Na pump towards the extracellular K concentration (Ko) is little affected.(ABSTRACT TRUNCATED AT 250 WORDS)