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

甲壳动物的心肌经常在兴奋性接点电位（EJPs）上产生分级的动作电位（尖峰）。龙虾心肌细胞的峰电位幅度随温度的降低而增加。我们推测，温度下降通过激活心肌膜上的Ca^<+2>通道而引起内向Ca^<+2>电流增加。在龙虾心脏的神经-肌肉制备物中检验了这一假设。用电压钳方法分析肌膜电流，肌内电流分为与峰电位对应的快电流、与EJPs对应的慢电流和冷却引起的持续电流。当加入Co^+2（10-20 mM）时，前者很容易消失，而后两者几乎不消失.四乙基铵（TEA：K^+通道阻断剂）可增加放电峰的下降速率。但冷却效果更好。因此，内向电流的逐渐增加可能部分归因于外向K^+电流的抑制，但冷诱导的尖峰不能仅用K^+通道的抑制来解释。内向快速电流被硝苯地平（20 mM）和百日咳毒素（PTX）阻断。这些结果表明，心肌细胞膜上存在L型Ca^+2通道，PTX敏感的GTP结合蛋白（G蛋白）可能与冷诱导的Ca ~（2+）电流有关。当G蛋白亚基GDP-β-S和GTP-γ-S被注射到心肌细胞中时，GTP-γ-S增强了Ca^+2电流，而GDP-β-S降低了Ca ^+2电流。心包激素（F1和proctolin）的肽组分经常在废除它们的制剂中再次诱导尖峰。因此，心包肽可以激活G蛋白。冷抑制G蛋白介导的Ca^<+2>通道关闭可能是Ca ^<+2>电流冷依赖性增加的原因。

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.