Influence of anoxia and dinitrophenol on Ca2+ efflux and phosphorylase a activity in rabbit colon smooth muscle

It was observed in earlier studies that when the phosphorylase alpha activity of rabbit colon smooth muscle was increased by anoxia or 2,4-dinitrophenol (DNP), the calcium content of the mitochondrial fraction decreased. Despite this, under basal conditions there was no significant increase in the Ca2+ content in the fraction consisting of microsomes and cytoplasm. In the present study it was therefore investigated whether mitochondrial Ca2+ released by anoxia and DNP is translocated from the smooth muscle cells into the extracellular fluid. The Ca2+ efflux from rabbit colon into a Ca2+-free Krebs-Ringer bicarbonate buffer was measured by using a Ca2+-selective electrode. Both anoxia and DNP (6.6 X 10(-5)M) increased the Ca2+ efflux from the smooth muscle cells. The local anaesthetic D-mepivacaine, at a concentration of 1 X 10(-3)M, reduced the increase in Ca2+ efflux and simultaneously enhanced the anoxic or DNP-induced rise in phosphorylase alpha activity. The replacement of external Na+ by choline was found to reduce the basal Ca2+ efflux and to moderately increase change in the Ca2+ efflux, but the increase in phosphorylase alpha activity was greater than in the physiological buffer containing 137 mM Na+. These observations support the suggestion that anoxia and DNP, by releasing Ca2+ from the mitochondria, increase the phosphorylase alpha activity of smooth muscle.