Adenylate cyclase in sarcoplasmic reticulum of skeletal muscle: distribution, orientation, and regulation

We found specific activity of adenylate cyclase (AC) to be as high in rat skeletal muscle sarcoplasmic reticulum (SR) as in sarcolemma (SL) (39 +/- 5 pmol/mg per min and 34 +/- 5 pmol/mg per min). Detection of AC in SR could not be due to SL contamination. Activity in SR was similar in triads (heavy SR) and longitudinal reticulum (light SR), despite virtual absence of surface membrane markers in preparations of light SR. Also, AC of SR and SL may differ biochemically. In the presence of 10(-5) M 5'-guanylylimidodiphosphate, 10(-4) M isoproterenol increased SL activity 508%, crude SR 46.4%, heavy SR 68.3%, light SR only 24.3%. SR activity was 50% higher at 0.32 micromolar Ca++ than at 1 nanomolar Ca++ (p less than 0.05); higher concentrations of Ca++ noncompetively inhibited activity (Ki 0.87 micromolar). In contrast, Ca++ monophasically inhibited SL activity. Permeabilization of SR vesicles with alamethacin indicated that AC is on the cytoplasmic surface of SR; its regulation by physiological changes in cytoplasmic Ca++ could influence SR Ca++ flux.