Pei J-M. Modulation of intracellular calcium transient in response to ␤-adrenoceptor stimulation in the hearts of 4-wk-old rats during simulated weightlessness. Modulation of intracellular calcium ([Ca 2ϩ ]i) transient in response to ␤-adrenoceptor stimulation in the hearts of hindlimb unweighted (HLU) rats during simulated weightlessness has not been reported. In the present study, we adopted the rat tail suspension for 4 wk to simulate weightlessness. Effects of simulated microgravity on

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... enoceptor responsiveness were then studied. Mean arterial blood pressure, left ventricular pressure (LVP), systolic function [maximum positive change in pressure over time (ϩdP/dt max)], and diastolic function [maximum negative change in pressure over time (ϪdP/dtmax)] were monitored during the in vivo experiment. ␤-Adrenoceptor density was quantitated by radioactive ligand binding. Single rat ventricular myocyte was obtained by enzymatic dissociation method. ϮdP/dtmax, myocyte contraction, intracellular [Ca 2ϩ ]i transient, and L-type calcium current in response to ␤-adrenoceptor stimulation with isoproterenol were measured. Compared with the control group, no significant changes were found in heart weight, body weight, and mean arterial blood pressure, whereas LVP and ϮdP/dtmax were significantly reduced. LVP and ϮdP/dtmax were significantly attenuated in the HLU group in response to isoproterenol administration. In the in vitro study, the ␤-adrenoceptor density was unchanged. Effects of isoproterenol on electrically induced single-cell contraction and [Ca 2ϩ ]i transient in myocytes of ventricles in HLU rats were significantly attenuated. The enhanced L-type Ca 2ϩ current elicited by isoproterenol in cardiomyocytes was significantly decreased in the HLU group. The above results indicate that impaired function of L-type Ca 2ϩ current and decreased [Ca 2ϩ ]i transient cause the depressed responsiveness of the ␤-adrenoceptor stimulation, which may be partially responsible for the depression of cardiac function. intracellular calcium concentration transient; L-type calcium current