Effect of inotropic agents on the calcium binding to isolated cardiac sarcolemma

Ca2+ binding to fragmented sarcolemma isolated from canine heart was measured by an ultracentrifugation technique. Two classes of binding site with dissociation constants of 2.0 . 10(-5) and 1.2 . 10(-3) M were identified. The capacities of the high- and low-affinity sites were 15 and 452 nmol/mg, respectively. These sites were not affected by treatment with neuraminidase. The effects of various cations and drugs on Ca2+ binding were studied. All cations tested inhibited Ca2+ binding with the following order of potency: trivalent greater than divalent greater than monovalent cations. The order of potency for the monovalent ions was: Na greater than K+ greater than Li+ greater than or equal to Cs+ and for the divalent and trivalent ions: La3+ greater than or equal to Mn2+ greater than Sr2+ greater than or equal to Ba2+ greater than Mg2+. 1 . 10(-3) M caffeine and 1 . 10(-8) M ouabain increased the capacity of the low-affinity sites to 1531 and 837 nmol/mg, respectively. 1 . 10(-7) M verapamil, acidosis (pH 6.4), 1 -10(-5) M Mn2+ and 1 . 10(-4) M ouabain depressed the capacity of the low-affinity sites to a range of 154--291 nmol/mg. The dissociation constants of the high- and low-affinity sites and the capacity of the high-affinity sites were not affected by these agents.