Does the activity of ecto-5'-nucleotidase depend on the membrane potential?

在缺血状态下，腺苷是由5‘-核苷酸酶催化5’-一磷酸腺苷脱磷生成的。我们利用灵活安装的微透析技术研究了钾离子对腺苷生产的影响。将微透析探针植入麻醉开胸大鼠左室心肌，然后用含5‘-AMP的Tyrode液灌流。结果表明，持续供应5’-AMP所测得的透析液中的腺苷水平反映了内源性外源性5‘-核苷酸酶的活性。当Tyrode液中KCI浓度从5.4 mM(对照组)逐步增加到140 mM时，透析液中的腺苷水平显著增加，且呈浓度依赖关系，而1 mM CSCI，AK~+通道阻滞剂的存在可使后者的作用消失。通过添加蔗糖(270 MM)使泰罗德溶液渗透浓度的等量增加不影响透析器的…更多的人吃了浓缩的腺苷。内向整流钾电流阻断剂Csci和BaCl2(20 MM)对豚鼠心室乳头肌的静息膜电位有明显的去极化作用。为了排除高浓度KCI增加腺苷产生是由于交感神经末梢去极化后去甲肾上腺素释放所致的可能性，我们在α1-受体阻滞剂30 MU的哌佐星存在下重复了同样的实验。即使在这种情况下，加入含140 mM KCl的溶液也能显著增加腺苷的产生，从而排除了去甲肾上腺素诱导的α_1受体介导的蛋白激酶C活性激活所介导的腺苷产生的可能性。这些结果表明，细胞外K~+浓度的增加使心肌细胞静息膜电位去极化，并增加了外膜-5‘-核苷酸酶的活性，最终增加了在体大鼠心脏腺苷的产生。较少

Under ischemia, adenosine is produced by enzymatic dephosphorylation of adenosine 5'-monophosphate (5'-AMP) by 5'-nucleotidase. We studied the effect of potassium ions on the production of adenosine, using flexibly mounted microdialysi techniques. The microdialysisi probe was implanted into the left ventricular myocardium of anesthetized open-chested rats, followed by a perfusion with Tyrode solution containing 5'-AMP.We clarified that the level of dialysate adenosine measured during continued supply of 5'-AMP reflects the activity of endogenous ecto-5'-nucleotidase. When the concentration of KCI in the Tyrode solution was increased stepwise from 5.4 mM (control) to up to 140 mM,the level of dialysate adenosine significantly increased, in a concentration-dependent manner and the latter effect was abolished in the presence of 1 mM CsCI,aK^+ channel inhibitor. Equivalent increases in osmotic concentration of the Tyrode solution, made by adding sucrose (270 mM) , did not affect the dialys … More ate adenosine concentration. In isolated guinea pig ventricular papillary muscles, the blockers of inward rectifier K^+ current (I_<K1>), i.e., CsCI and BaCI_2 (20mM) depolarized the resting membrane potential significantly. To rule out the possibility that the enhanced production of adenosine by high concentration of KCI is caused by the release of noradrenaline, secondary to the depolarization of sympathetic nerve terminals, we repeated the same experiments in the presence of 30 muM prazocin, an alpha^1-blocker. Even under this condition, introduction of 140 mM KC1 containing solution significantly enhanced the adenosine production, thereby dismissing the possibility of adenosine production mediated by noradrenaline-induced, alpha_1-receptor-mediated activation of protein kinase C activity.These results suggest that increases in extracellular K^+ concentration depolarizes the resting membrane potential of cardiomyocytes and increase the activity of ecto-5'-nucleotidase with eventual increases of adenosine production in in situ rat's hearts. Less