Increased presynaptic a-adrenoceptor-mediated regulation of noradrenaline release in the isolated perfused kidney of spontaneously hypertensive rats

1. Release of [3Hlnoradrenaline during periarterial nerve stimulation and its inhibition by the presynaptic a-adrenoceptor mechanism were studied in the isolated perfused kidney from spontaneously hypertensive and Wistar-Kyoto rats. 2. A frequency related vasoconstriction as well as 3H]noradrenaline release were observed over the stimulating range of 0.25-32 Hz in both the Wistar-Kyoto and spontaneously hypertensive rats. The spontaneously hypertensive rat kidneys exhibited both an increased vasoconstrictor response and a greater [ )HInoradrenaline release when compared with the Wistar-Kyoto rat kidneys. 3. Presynaptic inhibition of [3Hlnoradrenaline release was evaluated at 2 Hz by using the a-adrenoceptor agonist, tramazoline. Increasing concentrations of tramazoline from 2 x mol/l to 2 x lo-’ moll1 caused a dose-dependent decrease in the stimulus-induced release of [ 3H]noradrenaline in spontaneously hypertensive rats but not in Wistar-Kyoto rats. Only 2 x lo-’ mol/l tramazoline had an inhibitory effect in the Wistar-Kyoto rats. 4. These data indicate that noradrenaline release during sympathetic nerve stimulation is greater in the spontaneously hypertensive rat. The supersensitivity of presynaptic cradrenoceptors observed in spontaneously hypertensive rats may be a consequence of the greater noradrenaline release present in these animals.