Absence of effect of calcium antagonists on endothelium-dependent relaxation in rabbit aorta

The effect of chronic feeding of New Zealand White rabbits with nicardipine (60 mg kg-1 daily for 5 weeks) on the endothelium-dependent relaxation (EDR) to acetylcholine (ACh) was examined in vitro. The effect of acute exposure to nicardipine and diltiazem (10 mumol l-1) in the tissue bath was also examined. A bioassay system for endothelium-dependent relaxation factor (EDRF) in which a rabbit aortic ring with endothelium removed was used as recipient and a segment of rabbit aorta with endothelium as donor (producing EDRF in response to ACh) was developed. This system enabled the effect of nicardipine on the synthesis/release and on the relaxation to EDRF to be studied separately. The maximum relaxations to ACh in control and nicardipine-fed animals were 43.6 +/- 5.5 and 53.8 +/- 6.7% (mean +/- s.e. mean) of the contractile response to noradrenaline (NA, 1 mumol l-1) (n = 6, P greater than 0.05). Similarly the EDR to ACh was not significantly altered by acute exposure (30 min) to nicardipine or diltiazem. The maximum relaxations without and with nicardipine were 32.4 +/- 4.2% and 28.0 +/- 3.1% of the contraction to NA (1 mumol l-1) (n = 11, P greater than 0.05). The corresponding data for diltiazem were 42.1 +/- 5.7 and 36.4 +/- 7.3% respectively (n = 11, P greater than 0.05). Both calcium antagonists inhibited the contraction induced by potassium (100 mmol l-1). Nicardipine and diltiazem in concentrations of 100 mumol l-1 reduced the potassium-induced contraction to 33.0 +/- 9.0% and 53.8 +/- 6.7% of control respectively (n = 6, P less than 0.05). In the bioassay experiments the infusion of nicardipine on (a) the recipient tissue only and (b) the donor and the recipient tissue had no significant effect on the relaxant response observed in the recipient tissue when superfused with Krebs-bicarbonate buffer containing ACh via the donor tissue (n = 6, P greater than 0.05). These results indicate that nicardipine and diltiazem had no significant effect on synthesis/release and the relaxant response to EDRF in the rabbit aorta. Thus the translocation of Ca2+ accompanying the EDR to ACh in the rabbit aorta is likely to utilize Ca2+ channels not blocked by these calcium antagonists.