A perforated patch-clamp study of calcium currents and exocytosis in chromaffin cells of wild-type and α_1A knockout mice

Simultaneous recordings of inward whole-cell Ca²⁺ channel currents (I_Ca) and increments of capacitance as an indication of exocytosis (ΔCm), were performed in voltage-clamped single adrenal chromaffin cells from wild-type and α_1A subunit deficient mice, using the perforated-patch configuration of the patch-clamp technique. Using protocol #1 (one single Ca²⁺ 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 μM nifedipine, 1 μM ω-conotoxin GVIA, and 1 μM ω-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 ΔCm paralleled those of I_Ca. In α_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, ΔCm 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 Ca²⁺ channel subtype, L, N, P/Q, R. Ablation of the α_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 α_1A-deficient mice. This may be due to altered Ca²⁺ homeostasis in these mice, as compared to wild mouse chromaffin cells.