The evaluation of the N-type channel blocking properties of cilnidipine and other voltage-dependent calcium antagonists.

Sympathetic neurotransmission in tissues with intact sympathetic nerve arborization is extensively dependent on calcium influx via N-type calcium-channels. It was the objective of the present study to assess and compare the claimed sympatholytic effect of the 1,4-dihydropyridine compound cilnidipine with other voltage-dependent calcium-channel (VDCC) antagonists. We studied these compounds by means of three different models. In the rabbit isolated thoracic aorta, the alleged sympatholytic properties displayed by these compounds were evaluated in the noradrenaline spillover model. Additionally, the influence of cilnidipine on stimulation-induced constrictor responses was studied in the rat isolated tail artery (male Wistar rats, 250-300 g) in addition to its effect on noradrenaline-induced contractions. Finally, we studied the influence of cilnidipine and other calcium-channel blockers on stimulation-induced chronotropic responses, in order to address N- or L-type selectivity, in the pithed rat model (male Wistar rats, 260-320 g). Furthermore, we evaluated their effect on noradrenaline-induced tachycardia. In the isolated rabbit thoracic aorta preparation omega-conotoxin GVIA (0.1 microM) nearly abolished the sympathetic outflow caused by stimulation, whereas nifedipine (0.1 microM) and amlodipine (1 microM) did not influence the evoked noradrenaline release. Cilnidipine (1 microM) significantly attenuated the response by nearly 18% and mibefradil (1 microM) by c. 42%. The stimulation-induced constrictor response (prejunctional effect) in the rat isolated tail artery could be blocked by omega-conotoxin GVIA (0.5 and 1 microM). Cilnidipine (10 nm and 0.1 microM) significantly attenuated responses to stimulation by maximally 20%, whereas it did not influence the constrictor response to noradrenaline (postjunctional effect). The mean heart rate in the pithed rat model amounted to 309.3 +/- 3.6 beats/min (bpm). Electrical stimulation of the cardio-accelerator nerves (C7-Th1) resulted in an increase by 106.7 +/- 2.2 bpm. All antagonists studied, except for nifedipine, attenuated the chronotropic response to stimulation (P < 0.05). The rank order of sympatholytic efficacy was: omega-conotoxin GVIA (84.8%), mibefradil (75.1%), cilnidipine (43.0%) and amlodipine (34.8%). Noradrenaline (10 nmol/kg) increased the heart rate by 117.8 +/- 2.7 bpm. This chronotropic response was influenced equally well by the calcium-channels blockers as observed in the stimulation (prejunctional) experiment. In conclusion, the N-type channel blocking properties and thus sympatholytic effect of cilnidipine could be demonstrated in some (vascular) but not all (cardiac) models studied. At the level of the vasculature cilnidipine reduced the neurotransmitter release to electrical stimulation in both the noradrenaline spillover model and in the model of the rat isolated tail artery, respectively. For amlodipine and nifedipine no sympatholytic activity could be demonstrated. In the pithed rat model, we were unable to demonstrate a selective N-type blocking effect for the VDCC-antagonists.