Regunathan S, Reis D J, Wahlestedt C
Department of Neurology and Neuroscience, Cornell University Medical College, New York, NY 10021.
Biochem Pharmacol. 1992 Mar 17;43(6):1331-6. doi: 10.1016/0006-2952(92)90510-p.
In a membrane preparation of cultured bovine adrenal chromaffin cells, [3H]inositol hexakisphosphate ([3H]InsP6) was shown to bind specifically with a Kd of 90 nM and a Bmax 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.
