Abstract
The effects of dopamine receptor and alpha-adrenoceptor agonists and antagonists on the stimulation-evoked overflow of radioactivity from strips of dog saphenous vein previously loaded with [3H]-noradrenaline have been examined alone and in combination. In the presence of neuronal and extraneuronal catecholamine uptake inhibitors, noradrenaline (0.1-1 X 10(-6)M) and dopamine (0.01-1 X 10(-6)M) both inhibited the stimulation-evoked overflow of radioactivity. Sulpiride (1 X 10(-6)M) was without effect and prazosin (1 X 10(-7)M) had little effect on stimulation-evoked overflow but yohimbine enhanced it approximately 2 fold; the effect of yohimbine was similar at concentrations of 1 X 10(-7) and 1 X 10(-6)M. Sulpiride abolished the inhibitory effect of dopamine on stimulation-evoked overflow, but was without effect against noradrenaline. When allowance was made for the effects of yohimbine, alone, on overflow, yohimbine (1 X 10(-7)M) had no effect against dopamine and minimal effects against noradrenaline. A similar result was obtained when the concentration of yohimbine was increased to 1 X 10(-6)M. Prazosin did not antagonize the effect of noradrenaline. In the absence of the uptake inhibitors, clonidine (0.01-1 X 10(-5)M) inhibited stimulation-evoked overflow of radioactivity. Yohimbine (1 X 10(-6)M) was without effect on its own and antagonized the effects of clonidine at a concentration of 0.1 X 10(-5)M, but not at 0.01 or 1.0 X 10(-5)M. These findings suggest that dopamine inhibits overflow by stimulating presynaptic dopamine receptors on the terminals of the noradrenergic nerves supplying the dog saphenous vein.(ABSTRACT TRUNCATED AT 250 WORDS)
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