Regulation of the ATP dependent calcium uptake activity of heart sarcolemmal vesicles by endogenous cytosolic proteins

Abstract

In a previous study we described the inhibitory action of a cytosolic protein fraction from heart muscle on ATP-dependent Ca²⁺ uptake by sarcoplasmic reticulum; further, this inhibition was shown to be blocked by an inhibitor antagonist, also derived from the cytosol (Narayanan et al. Biochim Biophys Acta 735: 53–66, 1983). The present study examined the effects of the endogenous cytosolic Ca²⁺ transport inhibitor and its antagonist on ATP-dependent Ca²⁺ uptake by sarcolemmal vesicles isolated from rat and canine heart. The cytosolic inhibitor caused strong inhibition (up to 97%) of Ca²⁺ uptake by sarcolemma (SL); this inhibition could be reversed by the cytosolic inhibitor antagonist. Studies on the characteristics of inhibition revealed the following:

• a) Inhibition was dependent on the concentration of the inhibitor (50% inhibition with ≈ 80 μg inhibitor protein).

• b) The inhibitor reduced the velocity of Ca²⁺ uptake without appreciably influencing the apparent affinity of the transport system for Ca²⁺ but caused > 2-fold decrease in its apparent affinity for ATP.

• c) The rates of unidirectional passive Ca²⁺ release from actively Ca²⁺ loaded SL vesicles were not altered by low concentrations of the inhibitor (< 100 μg/ml) which were effective in producing marked inhibition of Ca²⁺ uptake; at higher concentrations (> 100 μg/ml), the inhibitor caused increase in the rates of passive Ca²⁺ release.

These findings demonstrate that the activity of the ATP-driven Ca²⁺ pump of cardiac SL can be regulated in vitro by endogenous cytosolic proteins.