A perforated patch-clamp study of calcium currents and exocytosis in chromaffin cells of wild-type and alpha(1A) knockout mice.
Simultaneous recordings of inward whole-cell Ca(2+) channel currents (I(Ca) ) and increments of capacitance as an indication of exocytosis (Delta(Cm)), were performed in voltage-clamped single adrenal chromaffin cells from wild-type and alpha(1A) subunit deficient mice, using the perforated-patch configuration of the patch-clamp technique. Using protocol #1 (one single Ca(2+) channel blocker per cell), to dissect the components of I(Ca), L channels contributed 43%, N channels 35% and P/Q channels 30% to the total I(Ca) of wild-type cells. Using protocol #2 (cumulative sequential addition of 3 microm nifedipine, 1 microm omega-conotoxin GVIA, and 1 microm omega-agatoxin IVA), L, N and P/Q channels contributed 40%, 34% and 14%, respectively, to I(Ca); an R component of around 11% remained. In wild-type mice the changes of Delta(Cm) paralleled those of I(Ca). In alpha(1A) deficient mice the L component of I(Ca) rose to 53% while the P/Q disappeared; the N and R components were similar. In these mice, Delta(Cm) associated to N and R channels did not vary; however, the P/Q component was abolished while the L component increased by 20%. In conclusion, exocytosis was proportional to the relative density of each Ca(2+) channel subtype, L, N, P/Q, R. Ablation of the alpha(1A) gene led to a loss of P/Q channel current and to a compensatory increase of L channel-associated secretion; however, this compensation was not sufficient to maintain the overall exocytotic response, that was diminished by 35% in alpha(1A) -deficient mice. This may be due to altered Ca(2+) homeostasis in these mice, as compared to wild mouse chromaffin cells.