Effects of calcium channel blockers on isolated carotid baroreceptors and baroreflex

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Abstract
Effects of the Ca antagonists, nifedipine and verapamil, on the carotid sinus baroreceptors and baroreflex were studied. The left carotid sinus region in dogs was vascularly isolated and filled with oxygenated physiological salt solution. Steady-state multiunit activity was recorded from the carotid sinus nerve for sinus pressures of 50-200 mm Hg after bathing the carotid sinus region in a solution containing no drug, 10 .mu.g/ml nifedipine (n = 6) or 5 .mu.g/ml verapamil (n = 5). The slopes of curves relating carotid sinus nerve activity (% of maximum control) to carotid sinus pressure were control, 0.81 .+-. 0.06; nifedipine, 1.29 .+-. 0.14: and verapamil, 0.48 .+-. 0.06%/mm Hg, indicating that nifedipine increased and verapamil decreased sensitivity of the carotid sinus baroreceptors. Additional curves with bilateral carotid sinus isolation (carotid sinus curves intact) indicated that nifedipine enhanced and verapamil attenuated carotid baroreflex control of renal sympathetic nerve activity. Pressure-volume curves generated in the isolated carotid sinus showed that effects on smooth muscle do not account for the opposing effects of the 2 Ca2+ antagonists. Omitting Ca2+ from the physiological solution resulted in increased carotid sinus nerve activity, an effect blocked by verapamil but not nifedipine. Verapamil, but not nifedipine, inhibited veratrine-induced (Na+-dependent) excitation of carotid baroreceptors. Excitatory effects of nifedipine on the carotid sinus baroreceptors are dependent on Ca2+ mechanisms; inhibitory effects of verapamil may be due mainly to interference with the inward Na+ current.