REGULATION OF CARDIAC CA2-NUCLEOTIDES UNDER NORMAL AND SIMULATED ISCHEMIC CONDITIONS( RELEASE CHANNEL (RYANODINE RECEPTOR) BY CA2+, H+, MG2+, AND ADENINE)

In myocardial ischemia, pH(i) and [ATP] fall, whereas the free [Ca2+] and [Mg2+] rise. The effects of these changes on cardiac Ca2+ release channel (ryanodine receptor) activity were investigated in [H-3]ryanod ine binding and single-channel measurements, using isolated membrane a nd purified channel preparations. In the absence of the two channel li gands Mg2+ and ATP, cardiac Ca2+ release channels were half-maximally activated at pH 7.4 by approximate to 4 mu mol/L cytosolic Ca2+ and ha lf-maximally inhibited by approximate to 9 mmol/L cytosolic Ca2+. Regu lation of channel activity by Ca2+ was modulated by Mg2+ and ATP. Sing le-channel activities were more sensitive to a change of cytosolic pH than SR lumenal pH. Reduction in lumenal and/or cytosolic pH from 7.3 to 6.5 and 6.0 resulted in decreased single-channel activities without a change in single-channel conductance. [H-3]Ryanodine binding measur ements also indicated that acidosis impairs cardiac Ca2+ release chann el activity. Mg2+ and adenine nucleotide concentrations regulated the extent of inhibition and the Ca2+ dependence of binding. In the presen ce of 5 mmol/L Mg2+ and 5 mmol/L beta, gamma-methyleneadenosine 5'-tri phosphate (AMPPCP, a nonhydrolyzable ATP analogue), the free [Ca2+] fo r half;maximal [H-3]ryanodine binding was increased from 1.9 mu mol/L at pH 7.3 to 36 mu mol/L at pH 6.5 and to 89 mu mol/L at pH 6.2. These results suggest that ionic and metabolic changes that might be expect ed to affect sarcoplasmic reticulum Ca2+ release channel activity in i schemic myocardium include an altered Ca2+ sensitivity of the channel, a fall in pH, and a loss of the high-energy adenine nucleotide pool, leading to an increased inhibition by Mg2+.