高钙对兔窦房结的负性变时作用

本文采用常规微电极技术在离体兔心窦房结标本上研究了细胞外钙离子浓度(Ca~(2 )]_0)对窦房结的变时性作用。结果显示,Ca~(2 )]_0的递增(1.1—5.5mM),对窦性周期(CL)大于400ms 的标本引起双相变时作用,但对CL小于400ms的标本却引起负性变时作用。阿托品(0.5mg/l)和心得安(0.2mg/l)对Ca~(2 )]_0 的变时作用无明显影响。随着Ca~(2 )]_0的递增,窦房结优势起搏细胞的动作电位幅度、最大舒张期电位和0期最大除极速度均降低,舒张期自动除极化斜率增加,起始电位上移,动作电位时程(APD)延长。高Ca~(2 )]_0时窦房结起搏细胞的有效不应期(ERP)延长、ERP/APD增大,ERP点的阈值提高(P<0.01)。 上述结果表明,高Ca~(2 )]_0引起窦房结的负性变时效应,这种作用不是通过交感和副交感神经的传递,而可能是 Ca~(2 )直接作用于窦房结起搏细胞引起其电活动改变的结果。

NEGATIVE CHRONOTROPIC EFFECT OF CALCIUM ON THE SINUS NODE OF RABBITS

The chronotropic effect of Ca~(2 )]_0 was studied in the isolated rabbit sinus nodeby means of the microelectrode recording technique. The results were as follows:(1) The chronotropic effect of Ca~(2 )]_0 depended on the sinus cycle length; in prepara-tions with long cycle length (>400ms), increase in Ca~(2 )]_0 from 1.1 to 5.5 mMproduced a biphasic chronotropic effect, whereas in preparations with short cyclelength (<400ms) high Ca~(2 )]_0 produced a negative chronotropic effect. (2) Atropineand propranolol did not apparently affect the chronotropic action of calcium. (3)When Ca~(2 )]_0 was increased, the dominant pacemaker cells showed decreases inthe amplitude of the action potential, the maximum diastolic potential and themaximum rate of depolarization, but the slope of diastolic depolarization was in-creased with an upward shift of the take-off potential and lengthening of actionpotential duration (APD). (4) At a still higher level of Ca~(2 )]_0 (4.4mM), theeffective refractory period (ERP), ERP /APD radio and the threshold at the endingpoint of ERP (P<0.01) were increased. The results suggest that the negative chronotropic effect of high Ca~(2 )]_0 onthe sinus node does not seem to be mediated through sympathetic and parasym-pathetic nerves, but rather through the direct action of the Ca~(2 ) upon the electricactivity of the pacemaker cells in the sinus node.