Altered regulation of calcium channels and exocytosis in single human pheochromocytoma cells

We established primary cultures of human pheochromocytoma chromaffin cells. We then tried to find what mechanism of their secretory apparatus could be altered to produce the massive release of catecholamines into the circulat ion and the subsequent hypertensive crisis observed inpatients suffering th is type of tumor. Their whole-cell Ca2+ channel currents could be pharmacol ogically separated into components similar to those found in normal human a drenal chromaffin cells: 20% L-type, 30% N-type, and 50% P/Q-type Ca2+ chan nels. However, modulation of the channels by exogenous or endogenous ATP an d opioids, via a G-protein membrane-delimited pathway, was deeply altered; some cells having no modulation or very little modulation alternated with o thers having normal modulation. This may be the cause of the uncontrolled s ecretory response, measured amperometrically at the single-cell level. Some cells secreted for long time periods and were insensitive to nifedipine (L -type channel blocker) or to omega-conotoxin MVIIC (N/P/Q-type channel bloc ker), while others were highly sensitive to nifedipine and partially sensit ive to omega-conotoxin MVIIC. Alteration of the autocrine/paracrine modulat ion of Ca2+ channels may lead to indiscriminate Ca2+ entry and exacerbate c atecholamine release responses in human pheochromocytoma cells.