ANALYSIS OF THE MEMBRANE PROPERTIES OF THE COLD RESPONSIVE CELLS PRESENT IN CRUSTACEAN HEARTS.

甲壳类动物心肌在兴奋连接电位（EJPs）上经常产生梯度动作电位（spike）。日本参心肌细胞的峰值振幅随温度的下降成比例地增加。我们假设温度下降通过激活心肌膜的Ca^<+2>通道，导致向内Ca^<+2>电流增加。这一假设在龙虾心脏的神经肌肉准备中得到了检验。用电压钳法对肌膜电流进行了分析。肌肉向内电流被分成与峰值相对应的快速电流；向ejp输送的缓慢电流和由冷却引起的持续电流。当Co^<+2> (10-20 mM)时，前者很容易消失，后者几乎不存在。四乙基铵（TEA：一种K^+通道阻滞剂）提高了尖峰下降速率。但冷却的作用更大。因此，向内电流的逐渐增加可能部分归因于向外K^+电流的抑制，但冷诱导的尖峰不能仅仅用K^+通道的抑制来解释。硝苯地平（20 mM）和百日咳毒素（PTX）阻断了向内的快速电流。这些数据表明，心肌膜具有l型Ca^<+2>通道，并且ptx敏感的gtp结合蛋白（G蛋白）可能与冷诱导Ca2+电流有关。将G蛋白亚基gtp - β - s和gtp - γ - s注入肌细胞后，gtp - γ - s增强了Ca^<+2>电流，而gtp - β - s则降低了Ca^<+2>电流。心包激素的肽成分（F1和proctolin）在制备过程中经常再次引起尖峰，使其消失。因此，心包肽可能激活G蛋白。Ca^<+2>电流的冷依赖性增加可能是由于G蛋白介导的Ca^<+2>通道关闭的冷抑制所致。

Crustacean cardiac muscles often generate graded action potentials (spikes) on excitatory junction potentials (EJPs). Amplitude of the spikes increased in proportion to falling temperature in myocardial cells of Panulirus japonicus. We have supposed that falling temperature causes inward Ca^<+2> currents to increase by activation of Ca^<+2> channels of the myocardial membrane. This hypothesis was examined in the nerve-muscle preparations of the lobster heart. The muscle membrane currents were analyzedwith the voltage clamp method.The muscle inward currents were divided into rapid currents corresponding to the spikes ; slow currents to the EJPs and sustained currents induced by cooling. The former easily disappeared and the latter two hardly did with an application of Co^<+2> (10-20 mM). Tetraethylammonium (TEA : a K^+ channel blocker) increased the falling rate of spike. But cooling did it more. Therefore, the graded increase of inward currents may be partly attributed to depression of the outward K^+ currents, but the cold-induced spiking cannot be explained only by suppression of the K^+ channels. The inward rapid currents were blocked by nifedipine (20 mM) and also pertussis toxin (PTX). These data suggest that the myocardial membrane has L-type Ca^<+2>channels and that PTX-sensitive GTP-binding proteins (G proteins) may concern the cold-inducing Ca2+currents. When the G protein subunits, GDP-beta-S and GTP-gamma-S, were injected into the myocytes, the GTP-gamma-S enhanced the Ca^<+2> currents and the GDP- beta-S reduced them. Peptide components of the pericardial hormones (F1 and proctolin) often induced the spikes again in the preparation abolishing them. Therefore, the pericardial peptides may activate the G proteins. Then the cold dependent increase of Ca^<+2> currents may be resulting from cold depression of G protein-mediated closing of the Ca^<+2>channels.