INHIBITORY EFFECT OF CILNIDIPINE ON VASCULAR SYMPATHETIC NEUROTRANSMISSION AND SUBSEQUENT VASOCONSTRICTION IN SPONTANEOUSLY HYPERTENSIVE RATS

We reported previously that cilnidipine inhibited increases in blood pressure and plasma norepinephrine (NE) level in response to cold stress in spontaneously hypertensive rats (SHRs). In the present study, we investigated the effect of cilnidipine on sympathetic neurotransmission and subsequent vasoconstriction in SHRs. In pithed SHRs, electrical sympathetic nerve stimulation (ESNS) elevated blood pressure, and this presser response was abolished by guanethidine. Cilnidipine at 10 mu g/kg, i.v. and phentolamine at 1 mg/kg, i.v. suppressed the presser response to ESNS by 28+/-6% and 67+/-3%, respectively. Neither nifedipine nor nicardipine inhibited it. The presser response to exogenous NE was not influenced by cilnidipine. alpha,beta-Methylene ATP inhibited the presser response to ESNS in the presence or absence of phentolamine. Cilnidipine also attenuated the phentolamine-resistant presser response to ESNS. In SHR mesenteric vasculatures preloaded with [H-3]-NE, cilnidipine (10(-7) M) as well as omega-conotoxin significantly inhibited the H-3 overflow evoked by periarterial nerve stimulation. In radioligand binding experiments, cilnidipine inhibited [I-125]-omega-conotoxin binding to rat synaptosomes, but it did not inhibit [H-3]-prazosin binding to rat cortex membranes. These results suggest that cilnidipine may reduce electrically stimulated NE release from the sympathetic nerve endings of SHR vasculatures probably through its N-type Ca channel blocking action and that cilnidipine may also inhibit the vasoconstriction induced by ATP released concomitantly during nerve stimulation.