Introduction In addition to nitric oxide (NO), it is thought that an endothelium-derived hyperpolarizing factor (EDHF) plays an important role in the relaxation of penile arteries. Recently, it has been shown that C-type natriuretic peptide (CNP) shows the characteristics of EDHF in systemic small arteries. Aim To investigate the mechanism involved in CNP-evoked vasodilatation and to address whether CNP is an EDHF in human penile resistance arteries. Methods Erectile tissue was obtained in connection with transsexual operations. Intracavernous penile resistance arteries were isolated and mounted in microvascular myographs for recording of isometric tension. Membrane potential was recorded by the use of a small glass electrode inserted in the smooth muscle layer. Main Outcome Measure In vitro evidence for hyperpolarization and vasorelaxation induced by CNP. Results Acetylcholine (ACh) and CNP hyperpolarized smooth muscle membrane potential in resting penile resistance arteries. In penile small arteries incubated with inhibitors of NO synthase and cyclooxygenase and contracted with phenylephrine, ACh and CNP evoked concentration-dependent relaxations with maximum of 56±6% and 71±6%, respectively. Addition of a combination of blockers of small- and intermediate-conductance calcium-activated K+ channels, apamin plus charybdotoxin, respectively, and a combination thought to block the smooth muscle response of EDHF-type relaxation, barium plus ouabain, markedly reduced ACh- and CNP-evoked relaxation. Iberiotoxin, a blocker of big-conductance calcium-activated K+ channels inhibited the vasorelaxant responses evoked by ACh and CNP. A selective natriuretic peptide receptor type C (NPR-C) agonist, C-atrial natriuretic factor4−23 (cANF4−23), induced relaxations with less maximum response compared to CNP. Conclusion The present findings suggest that CNP possesses the characteristics of an EDHF in human penile resistance arteries. By activation of natriuretic peptide receptor type B and NPR-C receptors, CNP causes relaxation by activation, respectively, of large-conductance calcium-activated K+ channels and Na+/K+-adenosine triphosphatase (ATPase), and barium-sensitive inward rectifier K+ channels. Modulation of the CNP pathway opens for new treatment modalities of erectile dysfunction.

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Midbrain slices containing the dorsal and medial raphe nuclei were prepared from rat brain in order to study serotonergic-GABAergic interaction. The slices were loaded with either [3H] serotonin or [3H]GABA, superfused and the electrically induced efflux of radioactivity was determined. The GABA(A) receptor agonist muscimol (3 to 30 microM) and the GABA(B) receptor agonist baclofen (30 and 100 microM) inhibited [3H]serotonin and [3H]GABA release. These effects of muscimol were reversed by the GABA(A) antagonists bicuculline (100 microM). The GABA(B) antagonist phaclofen (100 microM) also antagonized the baclofen-induced inhibition of [3H]serotonin and [3H]GABA release. Phaclofen by itself increased [3H]serotonin release but it did not alter [3H]GABA overflow. Muscimol (10 microM) and baclofen (100 microM) also inhibited [3H]serotonin release after depletion of GABAergic neurons by isoniazid pretreatment. These findings indicate the presence of postsynaptic GABA(A) and GABA(B) receptors located on serotonergic neurons. The 5-HT1A receptor agonist 8-OH-DPAT (0.01 to 1 microM) and the 5-HT1B receptor agonist CGS-12066A (0.01 to 1 microM) inhibited the electrically stimulated [3H]serotonin and [3H]GABA release. The 5-HT1A antagonist WAY-100135 (1 microM) was without effect on [3H]serotonin and [3H]GABA efflux by itself but it reversed the 8-OH-DPAT-induced transmitter release inhibition. During KCl (22 mM)-induced depolarization, tetrodotoxin (1 microM) did not alter the inhibitory effect of CGS-12066A (1 microM) on [3H]GABA release, it did blocked, however, the ability of 8-OH-DPAT (1 microM) to reduce [3H]GABA efflux. After depletion of raphe serotonin neurons by p-chlorophenylalanine pretreatment, CGS-12066A (1 microM) still inhibited [3H]GABA release whereas in serotonin-depleted slices, 8-OH-DPAT (1 microM) was without effect on the release. We conclude that reciprocal influence exists between serotonergic projection neurons and the GABAergic interneurons or afferents in the raphe nuclei and these interactions may be mediated by 5-HT1A/B and GABA(A/B) receptors. Both synaptic and non-synaptic neurotransmission may be operative in the 5-HTergic-GABAergic reciprocal interaction which may serve as a local tuning in the neural connection between cerebral cortex and midbrain raphe nuclei.

Large-conductance Ca 2+ -activated K + channels (BK Ca ), located on the vascular smooth muscle, play an important role in regulation of vascular tone. In penile corpus cavernosum tissue, opening of BK Ca channels leads to relaxation of corporal smooth muscle, which is essential during erection; however, there is little information on the role of BK Ca channels located in penile vascular smooth muscle. This study was designed to investigate the involvement of BK Ca channels in endothelium-dependent and endothelium-independent relaxation of human intracavernous penile arteries. In human intracavernous arteries obtained in connection with transsexual operations, change in isometric force was recorded in microvascular myographs, and endotheliumdependent [nitric oxide (NO) and endothelium-derived hyperpolarization (EDH)-type] and endothelium-independent (NO-donor) relaxations were measured in contracted arteries. In penile small arteries contracted with phenylephrine, acetylcholine evoked NO-and EDH-type relaxations, which were sensitive to iberiotoxin (IbTX), a selective blocker of BK Ca channels. Iberiotoxin also inhibited relaxations induced by a NO-donor, sodium nitroprusside. NS11021, a selective opener of BK Ca channels, evoked pronounced relaxations that were inhibited in the presence of IbTX. NS13558, a BK Ca -inactive analogue of NS11021, failed to relax human penile small arteries. Our results show that BK Ca channels are involved in both NO-and EDH-type relaxation of intracavernous penile arteries obtained from healthy men. The effect of a selective opener of BK Ca channels also suggests that direct activation of the channel may be an advantageous approach for treatment of impaired endothelium-dependent relaxation often associated with erectile dysfunction.Erection is a haemodynamic event where vasodilatation of intracavernous arteries is followed by increased arterial blood inflow to the corpora cavernosa. During this process, activation of parasympathetic pelvic nerves leads to arterial dilatation and to initiation of erection while flow stimulation of the endothelium in the penis contributes to the sustained vasodilatation during erection. Nitric oxide (NO) released from both parasympathetic nerves and endothelial cells is a key mediator in erection [1]. In addition to NO, prostanoids and a non-NO/ non-prostanoid endothelium-derived hyperpolarization (EDH) have also been suggested to play a role in endothelium-dependent relaxation in erectile tissue [2]. EDH induces vascular relaxation in human [3,4], horse [5], bovine [6] and rat [7] penile arteries. The nature of EDH is still unclear, but myoendothelial gap junctions [8], potassium ions [9], products of the cytochrome P450 pathway [10], C-type natriuretic peptide (CNP) [3,11] and hydrogen peroxide [12] have been suggested to mediate EDH and lead to smooth muscle hyperpolarization and vasodilatation. Both smooth muscle inward-rectifier K + channels, Na-K-ATPase, and large-conductance calcium-activated K + channels have been proposed to be in...