Characterization of prejunctional alpha-2 adrenergic receptors involved in modulation of adrenergic transmitter release in the isolated perfused rat kidney.

The subclassification of alpha-2 adrenergic receptors into A and B subtypes is based on radioligand binding and functional studies. Radioligand binding studies also have suggested the existence of C and D subtypes, which have only been described as binding sites. This study was designed to determine the subtype of prejunctional alpha-2 adrenergic receptor involved in inhibition of norepinephrine release from sympathetic nerves in the rat kidney. Electrically induced (0.25 Hz, 50 V, 0.5 msec, 10 pulses) fractional tritium overflow was measured in kidneys isolated from male Sprague-Dawley rats and prelabeled with [3H]norepinephrine. The alpha-2 receptor antagonists rauwolscine and yohimbine did not enhance fractional tritium overflow, suggesting the lack of autoinhibition at this frequency of stimulation. The alpha-2 receptor agonist, UK-14,304, inhibited electrically stimulated fractional tritium overflow with an ED50 of 4.85 +/- 0.35 nM. pA2 values for various alpha receptor antagonists against UK-14,304-induced inhibition of fractional tritium overflow were: rauwolscine, 8.8; yohimbine, 8.1; prazosin, 7.4; BAM 1303, 7.5; SKF 104078, 6.4; phentolamine, 8.7; WB 4101, 8.1; corynanthine, 6.1; and ARC-239, 7.2. The correlation coefficients and slopes of the regression lines between pA2 values of alpha receptor antagonists at the prejunctional alpha-2 adrenergic receptor and the reported pKi values obtained from radioligand binding studies were: 0.48 and 0.82; 0.61 and 0.58; 0.53 and 0.85; and 0.89 and 1.05 for the alpha-2A, B, C and D adrenergic receptors, respectively. These data suggest that the prejunctional alpha-2 adrenergic receptor modulating [3H]norepinephrine release in the rat kidney most closely resembles the alpha-2D adrenergic receptor characterized by radioligand binding studies.