The electrical and mechanical responses of the rabbit saphenous artery to nerve stimulation and drugs.

1. Electrical and mechanical responses to field stimulation (1-64 Hz, 0.5 ms supramaximal voltage) were recorded simultaneously in the rabbit saphenous artery. The electrical response consisted entirely of excitatory junction potentials (e.j.ps) which were abolished by alpha, beta methylene ATP (alpha, beta MeATP, 10(-6) M) and by tetrodotoxin (TTX, 10(-6) M) but were unaffected by the alpha 1-adrenoceptor antagonist, prazosin (10(-6) M). No additional electrical response was evoked by field stimulation, even in the presence of normetanephrine (NMN) and desmethylimipramine (DMI, each 10(-6) M), which block neuronal and extraneuronal uptake of noradrenaline (NA) respectively. Action potentials to field stimulation were produced only in the presence of tetraethylammonium (10(-3) M) which also enhanced the contraction. 2. Contractions to field stimulation were reduced (by some 50%) by prazosin (10(-6) M) and abolished by the additional presence of alpha, beta MeATP (10(-6) M), which blocks purinoceptors by desensitization, suggesting the involvement of both NA and an ATP-like substance in the contractile response. 3. Idazoxan (10(-6) M) which blocks prejunctional alpha 2-adrenoceptors, significantly increased the amplitude of both e.j.ps and the contraction to field stimulation (10 pulses, 1-4 Hz, 0.5 ms, supramaximal voltage). 4. NA (10(-2) M by pressure ejection) did not alter membrane potential even in the presence of NMN and DMI (each 10(-6) M). ATP (10(-2) M by pressure ejection) produced a concentration-dependent, alpha, beta MeATP-sensitive depolarization. 5. In tissues desensitized by constant infusion of alpha, beta MeATP (10(-6) M contractions to NA (10(-7) - 3 x 10(-5) M), histamine (10(-7) - 3 x 10(-5) M) and KCl (1-1.6 x 10(-2) M) were unaffected.6. Following restoration of the membrane potential, after an initial depolarization, alpha,beta MeATP (4 x 10-6M) did not change the amplitude of electrotonic hyperpolarizing current pulses significantly but abolished evoked ej.ps. The rates of recovery of evoked ej.ps and the depolarization to ATP (10-2M by pressure ejection) following desensitization to alpha,beta MeATP (10- 6 M) were comparable. These results suggest that the effects of a,fl MeATP are mediated selectively via receptors (purinoceptors).7. Suramin (10-3M) abolished ej.ps and the prazosin (10-6M), insenstive component of the contractile response and antagonized contractions to xfl MeATP (10-7_1i-5M), ATP (10-5_1i-3M), histamine (10- 7-3 x 10-SM) and 5-hydroxytryptamine (10-7-10-SM) but those to NA (10-7-10-SM) and KCI (1-1.6 x 10-2 M) were unaffected. Suramin is insufficiently selective, under these conditions, as a purinoceptor antagonist.