MOLECULAR MECHANISM OF THE ALPHA-1-ADRENOCEPTOR INDUCED CAVERNOUS SMOOTH-MUSCLE CONTRACTION

The relaxation of cavernous smooth muscle tissue is the essential step in the initiation of penile erection which is the therapeutical goal in the treatment of erectile dysfunction (ED) [10]. Cellular contracti lity is determined by changes of the free intracellular Ca2+-concentra tion [Ca2+](i) which is modulated by interactions of Ca2+-influx throu gh voltage-dependent Ca2+-channels (I-Ca), Ca2+-release from intracell ular, non-mitochondrial Ca2+-stores and Ca2+-extrusion systems [9,17]. Hormones, neurotransmitter and other mediators may affect cavernous s mooth muscle contractility by modulating these cellular compartments v ia second messengers [8,9,17]. The present study focuses on the intrac ellular mechanism of the alpha(1)-adrenoceptor induced cavernous smoot h muscle contraction by applying the patch-clamp technique combined wi th the Ca2+-sensitive fluorescence dye FURA II to enzymatically disper sed human cavernous smooth muscle myocytes. The alpha(1)-adrenoceptor agonist norepinephrine (NE) selectively stimulated I-Ca in a dose-depe ndent-and fully reversible fashion simultaneously elevating [Ca2+](i) up to 1.4 mu M. The threshold concentration was 10 nM (n = 7), EC(50) was 100 nM, maximal effects were observed in the micromolar range. Int racellular application of inositol 1,4,5-trisphosphate (IP3) which rel eases Ca2+ from the IP3-sensitive Ca2+-pool [1,13], imitated the NE ev oked responses. Moreover the NE induced contraction was abolished when the myocytes were pretreated with the intracellular Ca2+-release bloc kers ryanodine (200 mu M, n = 11), thapsigargin (0.1 mu M, n = 12) or low-molecular weight heparin (50 mg/ml, n = 9) [2,8,11,15,17]. These e xperimental results indicate a coupling of alpha(1)-adrenoceptors to t he phospholipase C (PLC) converting phosphoinositol 4,5-bisphosphate i nto diacylglycerol (DAG), an endogenous activator of the protein kinas e C (PKC), and IP3 which increases [Ca2+](i) by releasing Ca2+ from IP 3-sensitive Ca2+-stores [1]. The elevated [Ca2+](i) seems to be respon sible for the stimulation of I-Ca most likely due to phosphorylation o f voltage-dependent Ca2+-channels by the Ca2+-calmodulin dependent pro tein kinase (CCPK). The blockade of this pathway results in cavernous smooth muscle relaxation which could be of benefit for therapeutical a pplications in ED.