Molecular Mechanism of the Alpha-1 Adrenoceptor Induced Cavernous Smooth Muscle Contraction
19 March 2008 (online)
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) . Cellular contractility is determined by changes of the free intracellular Ca2+-concentration [Ca2+]i which is modulated by interactions of Ca2+-influx through voltage-dependent Ca2+-channels (ICa), Ca2+-release from intracellular, non-mitochondrial Ca2+-stores and Ca2+-extrusion systems [9, 17]. Hormones, neurotransmitter and other mediators may affect cavernous smooth muscle contractility by modulating these cellular compartments via second messengers [8, 9, 17]. The present study focuses on the intracellular mechanism of the α1-adrenoceptor induced cavernous smooth muscle contraction by applying the patch-clamp technique combined with the Ca2+-sensitive fluorescence dye FURA II to enzymatically dispersed human cavernous smooth muscle myocytes. The α1-adrenoceptor agonist norepinephrine (NE) selectively stimulated ICa in a dose-dependent and fully reversible fashion simultaneously elevating [Ca2+]i up to 1.4 μM. The threshold concentration was 10 nM (n = 7), EC50 was 100 nM, maximal effects were observed in the micromolar range. Intracellular application of inositol 1,4,5-trisphosphate (IP3) which releases Ca2+ from the IP3-sensitive Ca2+-pool [1, 13], imitated the NE evoked responses. Moreover the NE induced contraction was abolished when the myocytes were pretreated with the intracellular Ca2+-release blockers ryanodine (200 μM, n = 11), thapsigargin (0.1 μM, n = 12) or low-molecular weight heparin (50 mg/ml, n = 9) [2, 8, 11, 15, 17]. These experimental results indicate a coupling of α1-adrenoceptors to the phospholipase C (PLC) converting phosphoinositol 4,5-bisphosphate into diacylglycerol (DAG), an endogenous activator of the protein kinase C (PKC), and IP3 which increases [Ca2+]i by releasing Ca2+ from IP3-sensitive Ca2+-stores . The elevated [Ca2+]i seems to be responsible for the stimulation of ICa most likely due to phosphorylation of voltage-dependent Ca2+-channels by the Ca2+-calmodulin dependent protein kinase (CCPK). The blockade of this pathway results in cavernous smooth muscle relaxation which could be of benefit for therapeutical applications in ED.
Erectile dysfunction - Cavernous smooth muscle - [Ca2+]i - Fura II - Patch-clamp technique