The increased sympathetic neurotransmission in benign prostatic hyperplasia (BPH) results in a α1C-adrenoceptor-mediated increase in prostatic smooth muscle tone which seems to be responsible for the dynamic infravesical obstruction occurring in BPH. The prostatic smooth muscle contractions evoked by norepinephrine can be efficiently blocked by α1-adrenoceptor blockers. Moreover, an impressive number of clinical trials illustrated the beneficial results of α1-adrenoceptor blockers in the treatment of BPH. However, despite knowledge of α1-adrenergic neurotransmission and the clinical application of its blockade by selective α1-adrenoceptor antagonists, very little is known about the intracellular pathways involved in the regulation of prostatic smooth muscle contractility. To study the intracellular mechanism of the α1c-adrenoceptor-induced prostatic smooth muscle contraction, the patch-clamp technique in the whole-cell configuration mode combined with the Fura-II fluorescence technique was used in human, enzymatically isolated smooth muscle cells obtained from patients undergoing transurethral resection of the prostate because of symptomatic BPH. Furthermore changes in prostatic smooth muscle contractility were registered in organ bath experiments. Application of the selective α1-adrenoceptor agonist phenylephrine (PE) increased the L-type Ca2+-channel current (ICa) dose dependently from 8 up to 18.5 µA/cm2, simultaneously elevating the free cytoplasmic Ca2+ concentration ([Ca2+]i) up to 1.9 µM. Pretreating the myocytes with pertussis toxin, an exotoxin of Bordetella pertussis which inactivates GTP-binding proteins (G proteins) of the Gi and Go family by ADP ribosylation, reduced the PE-induced lea stimulation by 71.5 ± 5.6% (n = 21). Dialysis of the cytosol with the second messenger inositol-1,4,5-trisphosphate (IP3), which releases Ca2+ from intracellular non-mitochondrial, IP3-sensitive Ca2+ pools, imitated the PE-evoked responses. Pretreating the myocytes with the Ca2+-release blockers ryanodine (10–100 µM, n = 8), thapsigargin (0.1 µM, n = 11) or low-molecular weight heparin (n = 14) largely attenuated the PE-evoked responses. The experimental results suggest a coupling of α1-adrenoceptors to phospholipase C-converting phosphoinositol-4,5-bisphosphate into diacylglycerol, an endogenous activator of the protein kinase C and IP3 which releases Ca2+ from intracellular stores stimulating ICa via Ca2+-calmodulin-dependent protein kinase induced phosphorylation of voltage-dependent Ca2+ channels. This knowledge could be of interest for conservative treatment in symptomatic BPH.

Keywords:
Benign prostatic hyperplasia, Smooth muscle, α1-Adrenoceptors, Second messengers Ca2+
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© 1995 S. Karger AG, Basel
1995
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