The Ca2+ and Mg2+ dependence of Ca2+ uptake and respiratory function of porcine heart mitochondria. Probable physiological significance during the cardiac contraction-relaxation cycle.

The pCa (-log [Ca]) dependence of myofibrillar MgATPase, mitochondrial Ca2+ uptake, state 3 and state 4 respiration rates were measured under identical conditions in the presence of 2.5 m~ M&+. The pCa of each solution was maintained with 10 m~ EGTA in both sucrose and KC1-containing media. Myofibrillar MgATPase varied from 10-90% of maximal activation between pCa 6.2 and 5.4 in the sucrose medium and between pCa 6.5 and 5.5 in the KC1 medium. Within