Photoelectric measurement of neurogenic vasoconstriction in jejunal branches of the rabbit mesenteric artery reveals the presence of presynaptic opioid delta-receptors.

The perivascular nerves of rabbit mesenteric arteries were stimulated with 15 pulses at 2 Hz, and decreases in external diameter were measured by means of a photoelectric device. Both extra- and intraluminally added [Met5]enkephalin 1 mumol/l depressed vasoconstriction, although with the second mode of application a larger inhibition occurred. Therefore, in the subsequent experiments all opioids were added into the lumen. [Met5]enkephalin 0.1 mumol/l had no effect. [D-Pen2, L-Pen5]enkephalin 3 mumol/l was less potent than [Met5]enkephalin 1 mumol/l. ICI 174864 1 mumol/l was also without effect when given alone, but antagonized the action of [Met5]enkephalin 1 mumol/l. Ethylketocyclazocine, dynorphin A(1-13), normorphine and DAGO, all 1 mumol/l, were ineffective. [Met5]enkephalin 1 mumol/l did not change the vasoconstriction evoked by the application of noradrenaline (0.1-3 mumol/l). It is concluded that in the mesenteric artery action potential-induced transmitter release, and in consequence vasoconstriction can be inhibited by the activation of presynaptic opioid delta-receptors.