Control of cytosolic free calcium by intracellular organelles in bovine adrenal glomerulosa cells. Effects of sodium and inositol 1,4,5-trisphosphate

The regulation of cytosolic free Ca2+ concentration (Ca2+]c) by intracellular organelles was studied in permeabilized bovine adrenal glomerulosa cells. Two compartments, with distinct characteristics, were able to pump Ca2+. A first pool, sensitive to ruthenium red and presumably mitochondrial, required respiratory chain substrates to maintain Ca2+]c around 700 nM. Ca2+ efflux from this compartment was activated by Na+ (ED50 = 5 mM). Inositol 1,4,5-trisphosphate (IP3) had no effect on this pool. A second nonmitochondrial pool required ATP to lower Ca2+]c to about 200 nM and released Ca2+ transiently upon addition of IP3. When the two systems were allowed to work simultaneously, the nonmitochondrial pool regulated Ca2+]c and IP3 released Ca2+ in a concentration-dependent manner (EC50 = 0.6 microM). Under these conditions the mitochondria seemed Ca2+ depleted. Upon repeated stimulations with IP3, a marked attenuation of the response was observed. This phenomenon was due to Ca2+ sequestration by a nonmitochondrial IP3-insensitive pool. Neither dantrolene (200 microM) nor 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (10 microM) were able to abolish IP3-induced Ca2+ release, though both compounds efficiently inhibited aldosterone production in intact cells stimulated with angiotensin II (10 nM) or K+ (12 mM). These results suggest that in permeabilized adrenal glomerulosa cells: the nonmitochondrial pool is responsible for buffering Ca2+]c and for releasing Ca2+ in response to IP3; at resting Ca2+]c levels, the mitochondria appear Ca2+ depleted; when Ca2+]c rises above their set point, the mitochondria accumulate Ca2+ as a function of Na+]c; 4) the mitochondria are not involved in the desensitization mechanism of the response to IP3.