Lara B, Gandía L, Martínez-Sierra R, Torres A, García A G
Departamento de Farmacología, Facultad de Medicina, Servicio de Farmacología Clínica, Hospital Reina Sofía, Universidad de Córdoba, Avda. Córdoba, Spain.
Pflugers Arch. 1998 Mar;435(4):472-8. doi: 10.1007/s004240050541.
This study uses a new strategy to investigate the hypothesis that, of the various Ca2+ channels expressed by a neurosecretory cell, a given channel subtype is coupled more tightly to the exocytotic apparatus than others. The approach is based on the prediction that the degree of inhibition of the secretory response by various Ca2+ channel blockers will differ at low (0.5 mM) and high (5 mM) extracellular Ca2+ concentrations ([Ca2+]o). So, at low [Ca2+]o the K+-evoked catecholamine release from superfused bovine chromaffin cells was depressed 60-70% by 2 microM omega-agatoxin IVA (P/Q-type Ca2+ channel blockade), by 3 microM omega-conotoxin MVIIC (N/P/Q-type Ca2+ channel blockade), or by 3 microM lubeluzole (N/P/Q-type Ca2+ channel blockade); in high [Ca2+]o these blockers inhibited the responses by only 20-35%. At 1-3 microM omega-conotoxin GVIA (N-type Ca2+ channel blockade) or 3 microM furnidipine (L-type Ca2+ channel blockade), secretion was inhibited by 30 and 50%, respectively; such inhibitory effects were similar in low or high [Ca2+]o. Combined furnidipine plus omega-conotoxin MVIIC, omega-agatoxin IVA or omega-conotoxin GVIA exhibited additive blocking effects at both Ca2+ concentrations. The results suggest that Q-type Ca2+ channels are coupled more tightly to exocytotic active sites, as compared to L-type channels. This hypothesis if founded in the fact that external Ca2+ that enters the cell through a Ca2+ channel located near to chromaffin vesicles will saturate the K+ secretory response at both [Ca2+]o, i.e. 0.5 mM and 5 mM. In contrast, Ca2+ ions entering through more distant channels will be sequestered by intracellular buffers and, thus, will not saturate the secretory machinery at lower [Ca2+]o.
在神经分泌细胞表达的各种Ca2+通道中，某一特定通道亚型与胞吐装置的偶联比其他通道更为紧密。该方法基于这样的预测：在低（0.5 mM）和高（5 mM）细胞外Ca2+浓度（[Ca2+]o）下，各种Ca2+通道阻滞剂对分泌反应的抑制程度会有所不同。因此，在低[Ca2+]o时，2 microM ω-芋螺毒素IVA（P/Q型Ca2+通道阻滞）、3 microM ω-芋螺毒素MVIIC（N/P/Q型Ca2+通道阻滞）或3 microM鲁贝唑（N/P/Q型Ca2+通道阻滞）可使灌流的牛嗜铬细胞中K+诱发的儿茶酚胺释放降低60 - 70%；在高[Ca2+]o时，这些阻滞剂仅抑制反应20 - 35%。在1 - 3 microM ω-芋螺毒素GVIA（N型Ca2+通道阻滞）或3 microM福尼地平（L型Ca2+通道阻滞）作用下，分泌分别被抑制30%和50%；在低或高[Ca2+]o时，这种抑制作用相似。福尼地平与ω-芋螺毒素MVIIC、ω-芋螺毒素IVA或ω-芋螺毒素GVIA联合使用在两种Ca2+浓度下均表现出相加的阻断作用。结果表明，与L型通道相比，Q型Ca2+通道与胞吐活性位点的偶联更为紧密。这一假说基于以下事实：通过位于嗜铬小泡附近的Ca2+通道进入细胞的细胞外Ca2+在两种[Ca2+]o（即0.5 mM和5 mM）时都会使K+分泌反应饱和。相反，通过较远通道进入的Ca2+离子将被细胞内缓冲剂螯合，因此在较低[Ca2+]o时不会使分泌机制饱和。
