EXAMINATION OF THE ROLE OF INHIBITION OF CYCLIC-AMP IN ALPHA(2)-ADRENOCEPTOR MEDIATED CONTRACTIONS OF THE PORCINE ISOLATED PALMAR LATERAL VEIN

1 We have examined the effect of elevation of cellular adenosine 3':5' -cyclic monophosphate (cyclic AMP) on alpha(1)- and alpha(2)-adrenocep tor-mediated contraction of the isolated palmar lateral vein of the pi g. Cellular cyclic AMP was increased by either inhibition of phosphodi esterase by rolipram, or direct activation of adenylyl cyclase by fors kolin. 2 Noradrenaline (1 nM-10 mu M) caused concentration-dependent c ontractions of the porcine isolated palmar lateral vein (pD(2) 7.32 +/ - 0.07, n = 10). The selective alpha(1)-adrenoceptor antagonist, prazo sin (0.1 mu M) and the selective alpha(2)-adrenoceptor antagonist, rau wolscine (1 mu M) caused a 10 fold rightward displacement of the conce ntration-response curve and a combination of the two antagonists cause d a 200 fold rightward displacement of the concentration-response curv e. The selective alpha(2)-adrenoceptor agonist, UK-14304, also produce d concentration-dependent contractions of the palmar lateral vein (pD( 2) 7.70 +/- 0.15, n = 5), but the maximum response was 55.5 +/- 7.6% ( n = 5) of that produced by noradrenaline. Prazosin (0.1 mu M) failed t o affect responses to UK-14304 but rauwolscine, 1 mu M, caused a 200 f old rightward displacement. The estimated pK(B) value for rauwolscine (8.28 +/- 0.19, n = 10) is consistent with inhibition of alpha(2)-adre noceptors. Thus, the porcine isolated palmar lateral vein has a popula tion of alpha(1)- and alpha(2)-adrenoceptors capable of producing a co ntraction. 3 Rolipram, 10 mu M, and forskolin, 1 mu M, caused a 2-3 fo ld rightward displacement of the noradrenaline concentration-response curve (CRC), but 1,9-dideoxyforskolin, 1 mu M, a forskolin analogue wh ich does not activate adenylyl cyclase, failed to produce a significan t inhibition of noradrenaline-induced contractions. The combination of forskolin (1 mu M) and rolipram (10 mu M) were additive, producing a 20 fold rightward displacement of the noradrenaline CRC. 4 Responses t o noradrenaline were similarly affected by a combination of rolipram ( 10 mu M) and prazosin (0.1 mu M) (isolation of alpha(2)-adrenoceptors) and the combination of rolipram (10 mu M) and rauwolscine (1 mu M) (i solation of alpha(1)-adrenoceptors), resulting in a 100 fold rightward displacement of the noradrenaline CRC. Although forskolin inhibited b oth alpha(1)- and alpha(2)-adrenoceptor-mediated contractions, the eff ects produced were not similar. In particular, noradrenaline, 0.3-3 mu M, produced a significant contraction in the presence of forskolin (1 mu M) and prazosin (0.1 mu M) (an alpha(1)-adrenoceptor-mediated resp onse) but not in the presence of forskolin (1 mu M) and rauwolscine (1 mu M) (an alpha(1)-adrenoceptor-mediated response). 5 Five minute exp osure to either rolipram (10 mu M) or forskolin (1 mu M) elevated [H-3 ]-cyclic AMP of the porcine isolated palmar lateral vein by approximat ely 70% and 150-200%, respectively. Neither noradrenaline (1 nM-100 mu M) nor UK-14304 (1 nM-100 mu M) affected basal levels of [H-3]-cyclic AMP, but both produced a concentration-dependent inhibition of forsko lin-stimulated [H-3]-cyciic AMP accumulation with a pK(i) of 7.43 +/- 0.1 (n = 3) and 7.97 +/- 0.18 (n = 3), respectively. The effect of nor adrenaline against forskolin-stimulated [H-3]-cyclic AMP accumulation was reversed by rauwolscine (1 mu M) but not by prazosin (0.1 mu M). I n contrast, alpha(2)-adrenoceptor activation did not affect rolipram-i nduced elevation of [H-3]-cyclic AMP. 6 These findings indicate that a lpha(2)-adrenoceptor contractions of the porcine isolated palmar later al vein are not produced by reduction in cellular cyclic AMP per se. I t is proposed that this response involves a novel signal transduction mechanism. However, when cellular cyclic AMP has been elevated by agen ts that stimulate adenylyl cyclase, rather than through inhibition of phosphodiesterase, the ability of alpha(2)-adrenoceptors to inhibit cy clic AMP formation may be of functional importance in vascular smooth muscle.