Regulation of intracellular Ca²⁺ release in corpus cavernosum smooth muscle: synergism between nitric oxide and cGMP

Abstract

]i was reversibly inhibited by 27 Ϯ 7% (n ϭ 21, P Ͻ 0.005) in rat and by 55 Ϯ 15% (n ϭ 9, P Ͻ 0.01) in human SMCs. SNAP and SIL also reduced the contractile response to PE. To investigate the mechanism, we applied mediators alone or in combination. The soluble guanylyl cyclase inhibitor ODQ reduced the effect of SNAP and SIL. SIL, cGMP analogs, and NO donors without SIL did not reduce the PE-induced rise of [Ca 2ϩ ]i. However, the combination of 8-bromocGMP with SNAP reduced the Ca 2ϩ peak by 42 Ϯ 9% (n ϭ 22, P Ͻ 0.01). Our results demonstrate that NO and cGMP act synergistically to reduce Ca 2ϩ release from intracellular stores. Reduction of intracellular Ca 2ϩ release may contribute to relaxation of the corpus cavernosum, leading to erection. calcium stores; nitric oxide; sildenafil citrate; inositol 1,4,5-trisphosphate receptor THE TONE OF VASCULAR SMOOTH MUSCLE in the corpus cavernosum regulates penile tumescence: tonic contraction maintains the flaccid state, and relaxation leads to erection. Contractility of penile smooth muscle cells (SMCs) is modulated by various neurotransmitters and locally produced vasoactive substances. Considerable evidence suggests that norepinephrine is primarily responsible for tonic contraction of corpus cavernosum SMCs through activation of ␣-adrenergic receptors (1). Agonist binding to ␣-adrenergic receptors activates a G proteincoupled pathway increasing intracellular inositol 1,4,5-trisphosphate (IP 3 ) and activation of IP 3 receptors, followed by Ca 2ϩ release (3). Nitric oxide (NO), which is released by both endothelial cells and nonadrenergic, noncholinergic nerves, leads to relaxation of corpus cavernosum and is necessary for erection (6,28). NO has a variety of cellular effects including direct effects on ion channels and activation of soluble guanylyl cyclase (sGC), which converts GTP to cGMP (4, 26). cGMP in turn activates cGMP-dependent ion channels, cGMP-dependent protein kinase (PKG), and cGMP-regulated phosphodiesterases (PDEs). It has been suggested that, in vascular smooth muscle, most of the cGMP effects are mediated by PKG, because in PKG-1-deficient mice, aortic and corpus cavernosum smooth muscles fail to relax upon activation of the NO/cGMP pathway (16,27). PKG has a variety of effects in smooth muscle, including inhibition of the IP 3 receptor (30) and regulation of the Ca 2ϩ sensitivity of contractile proteins (23). Specific PKG mechanisms contributing to the regulation of SMC tone almost certainly vary from tissue to tissue, and the identity of specific targets involved in relaxation of corpus cavernosum SMCs remains uncertain.PDE types 2, 3, 5, and 11 are expressed in corpus cavernosum SMCs, although the main PDE activity is due to PDE5, which hydrolyzes cGMP (5, 10, 25). Sildenafil citrate (Viagra) acts by enhancing NO-mediated smooth muscle relaxation by competitive inhibition of PDE5, thereby maintaining elevated intracellular cGMP levels (2, 5).We investigated the effect of NO on adrenergically stimulated changes in intracellular Ca 2ϩ concentrat...

Hypercontractility and impaired sildenafil relaxations in the BKCachannel deletion model of erectile dysfunction

“…No difference was observed in the overall relaxation through the VDCC inhibition. The small total relaxation of ϳ25% is consistent with other reports where transient PE-induced Ca 2ϩ influx was not reduced by removing extracellular Ca 2ϩ but dependent on release from intracellular Ca 2ϩ stores (43). Thus the observed increase of force oscillations is very likely due to membrane potential instability and increased opening of VDCCs but not due to a change in Ca 2ϩ channel expression in CCSM strips from Slo Ϫ/Ϫ mice or from Slo ϩ/ϩ mice in the presence of IBTX.…”

Rho kinase is involved in Ca2+ entry of rat penile small arteries

“…In rabbit CC, no significant inhibition by Y-27632 of the Ca 2ϩ entry evoked by Phe was found (29). Therefore, the present results first showing a role for Rho kinase in the modulation of Ca 2ϩ handling in penile small arteries confirm the differences in the intracellular Ca 2ϩ signaling mechanisms between trabecular and arterial penile smooth muscle (29,34,36).…”

“…Whereas Rho kinase is involved in the Ca 2ϩ sensitization of the contractile proteins in CC smooth muscle (36), it is unclear whether this kinase plays a role in the Ca 2ϩ handling of penile arteries. In small arteries, Rho kinase has recently been suggested to regulate smooth muscle Ca 2ϩ entry (9).…”

Physiological regulation of penile arteries and veins

“…47 There is a minor role for either intracellular Ca 2 þ mobilization or storeoperated Ca 2 þ (SOC) entry, in contrast to that reported for rat and human CC, where a significant contribution of Ca 2 þ from intracellular stores to phenylephrine-induced contractions has been reported. 48 However, a capacitative Ca 2 þ entry through SOCs not coupled to contraction but probably related to non-contractile functions of vascular smooth muscle cells, such as protein and gene expression, 49 is present in penile small arteries. Despite the large extracellular Ca 2 þ dependence of penile vasoconstriction, there are also potent mechanisms of Ca 2 þ sensitization of the contractile proteins involving several kinases such as protein kinase C (PKC), tyrosine kinases (TKs) and Rho kinase (RhoK), 47 which suggests that Ca 2 þ entry and Ca 2 þ sensitization may cooperate to elicit vasoconstriction upon a 1 -adrenergic receptor stimulation.…”