Alpha-1 receptor mediated smooth muscle regulation in benign prostatic hyperplasia

Symptoms in benign prostatic hyperplasia (BPH) are partially caused by an increased prostatic smooth muscle tone. This tone depends on extra- and intracellular Ca2+ stores and is regulated by various second messenger pathways. We investigated the role of intracellular Ca2+ stores and cyclic 3'-5' adenosine monophosphate (cAMP) in BPH. Contractions elicited by the specific alpha 1-receptor agonist phenylephrine (PE) were inhibited by the selective alpha 1-receptor antagonists prazosin and YM 617. To elucidate the contribution of intracellular Ca2+ stores to the alpha 1-receptor induced contraction nifedipine, a blocker of voltage dependent L-type Ca2+ channels (VDCC) was applied and found to inhibit the PE induced contractions up to 65%. To further confirm the participation of intracellular Ca2+ stores, we applied ryanodine (10 microM) which reduced the alpha 1-receptor mediated contractions up to 80%. The remaining contraction was sensitive to nifedipine. The cAMP pathway mediating smooth muscle relaxation by regulating intracellular Ca2+ concentrations ([Ca2+]i) was also investigated. Nonspecific phosphodiesterase (PDE) inhibitors such as papaverine (0.5 mM) and theophylline (1 mM) and the specific PDE inhibitor milrinone (0.5 mM), all of which prevent degradation of cAMP, suppressed the PE induced contractions by 82%, 91% and 68%, respectively. Forskolin (50 microM), an activator of adenylylecyclase (AC), inhibited the PE induced contractions by 83%. The membrane permeable cAMP analog, N6-2'-0-dibutyryladenosine derivative (dBcAMP) also reduced the PE induced response by 70%.