Thiol-reactive agents biphasically regulate inositol 1,4,5-trisphosphate binding and Ca2+ release activities in bovine adrenal cortex microsomes

Within all endocrine cells, the inositol 1,4,5-trisphosphate (InsP(3)) receptor plays an important role in regulation of the intracellular Ca2+ concentration. In the present study we showed that a single short-term treatment with either N-ethylmaleimide (known to decrease InsP(3) receptor activity) or thimerosal (known to increase InsP(3) receptor activity) caused time-dependent biphasic effects on the InsP(3) binding activity of bovine adrenal cortex microsomes. The early potentiating effect of thiol-reactive agents translated into a 2-fold increase in binding affinity and Ca2+ release efficiency. The late dampening effect of thiol-reactive agents translated into a continuous reduction of the maximal binding capacity of the microsomes with a concomitant decrease in Ca2+ release efficiency. Under these conditions, Western blot analyses demonstrated that the level of InsP(3) receptor protein was not modified. Sequential treatments with thimerosal and the reducing agent dithiothreitol showed that the InsP(3) receptor can readily oscillate between high and low affinity states that are related to its alkylation state. Our results suggest a common mode of action of thiol-reactive agents on the InsP(3) receptor. These results also support the contention that cellular mechanisms of thiol group modification could play important roles in regulation of the intracellular Ca2+ concentration.