Specific binding of inositol hexakisphosphate (phytic acid) to adrenal chromaffin cell membranes and effects on calcium-dependent catecholamine release

In a membrane preparation of cultured bovine adrenal chromaffin cells, [³H]inositol hexakisphosphate ([³H]InsP₆) was shown to bind specifically with a k_d of 90 nM and a B_max of 700 fmol/mg protein. The Hill coefficient was not significantly different from unity. The association of [³H]InsP₆ was slow, with equilibrium binding being reached within 10 min. The dissociation of [³H]InsP₆ showed monophasic kinetics. In [³H]InsP₆ competition binding experiments, we found that reduction in the number of phosphorylated sites in inositol resulted in a gradual loss of binding potency. In intact bovine adrenal chromaffin cells, InsP₆ elicited a concentration-dependent facilitation of ⁴⁵C²⁺ influx along with the release of the catecholamines, epinephrine and norepinephrine. The latter responses were slower and longer-lasting than responses to depolarizing stimuli, such as nicotine and high K⁺. The catecholamine release required the presence of extracellular Ca²⁺. In good agreement with the binding studies, lower inositol phosphates displayed reduced secretagogue potency. In conclusion, in bovine adrenal chromaffin cells, InsP₆ appeared to bind to specific sites and elicited Ca²⁺ influx and catecholamine release.