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 (a _Na ⁱ ). During activation of the Na pump a transient outward current (ΔI) anda _Na ⁱ decline exponentially with an identical time constant (τ). The transient outward current and the decline ina _Na ⁱ are blocked by 10⁻⁴ M dihydroouabain (DHO). Lowering the temperature from 42°C to 17°C prolongs τ. 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 τ varies within the temperature range studied. The Q₁₀ amounts to ∼1.2 between 42°C and 35°C, to ∼2.4 between 35°C and 22°C and to ∼2.1 between 35°C and 17°C. Correspondingly the activation energy of the active Na transport is not constant between 42°C and 17°C. It is calculated to be 3.4 kcal/mol between 42°C and 35°C, 15.9 kcal/mol between 35°C and 22°C and 12.4 kcal/mol between 35°C and 17°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 (K_o) is little affected. The unidirectional active Na efflux of a fibre as a function of the intracellular Na concentration (Na_i) at 35°C and 22°C is derived from the experemental data. The relationship is linear over the narrow Na_i range studied but seems to be more complex when a wider Na_i range is considered.