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Abstract
Efonidipine hydrochloride is a dual Ca channel blocker that inhibits T-type Ca channels, which are localized in the sinoatrial node and are involved in the pacemaker mechanism of the heart, as well as L-type Ca channels. In the present study, the effect of efonidipine hydrochloride in inhibiting an increase in heart rate along with changes in circulation during mental and physical stress was examined using normotensive volunteers. A mental arithmetic test caused significant increases in heart rate and blood pressure, which were significantly inhibited 3 hours after oral administration of 40 mg of efonidipine hydrochloride but not at 1 week after cessation of administration. In contrast, the plasma norepinephrine, epinephrine, renin activity, and aldosterone levels following the test were unchanged at both 3 hours and 1 week after administration of efonidipine hydrochloride. Physical stress in the form of cold by immersing one hand in ice water for 2 minutes induced similar increases in the heart rate and blood pressure, which were significantly reduced at 3 hours but not at 1 week after administration of efonidipine hydrochloride. The plasma levels of the humoral factors previously described following the physical stress were unchanged at either 3 hours or 1 week after administration of efonidipine hydrochloride. These results suggest that efonidipine hydrochloride inhibits increases in heart rate and blood pressure due to stress, presumably by blocking T-type Ca channels rather than by inhibiting the sympathetic nervous system. Therefore, efonidipine hydrochloride is expected to be a Ca antagonist that exhibits a strong cardioprotective effect.