Influence of denervation on neurogenic inhibitory response of corpus cavernosum and nitric oxide synthase histochemistry

The Acute Effects and Mechanism of Ketamine on Nicotine-Induced Neurogenic Relaxation of the Corpus Cavernosum in Mice

The present study aimed to investigate the acute effects and the mechanism of ketamine on nicotine-induced relaxation of the corpus cavernosum (CC) in mice. This study measured the intra-cavernosal pressure (ICP) of male C57BL/6 mice and the CC muscle activities using an organ bath wire myograph. Various drugs were used to investigate the mechanism of ketamine on nicotine-induced relaxation. Direct ketamine injection into the major pelvic ganglion (MPG) inhibited MPG-induced increases in ICP. D-serine/L-glutamate-induced relaxation of the CC was inhibited by MK-801 (N-methyl-D-aspartate (NMDA) receptor inhibitor), and nicotine-induced relaxation was enhanced by D-serine/L-glutamate. NMDA had no effect on CC relaxation. Nicotine-induced relaxation of the CC was suppressed by mecamylamine (a non-selective nicotinic acetylcholine receptor antagonist), lidocaine, guanethidine (an adrenergic neuronal blocker), N^w-nitro-L-arginine (a non-selective nitric oxide synthase inhibitor), MK-801, and ketamine. This relaxation was almost completely inhibited in CC strips pretreated with 6-hydroxydopamine (a neurotoxic synthetic organic compound). Ketamine inhibited cavernosal nerve neurotransmission via direct action on the ganglion and impaired nicotine-induced CC relaxation. The relaxation of the CC was dependent on the interaction of the sympathetic and parasympathetic nerves, which may be mediated by the NMDA receptor.

Characterization of nitrergic function in monkey penile erection in vivo and in vitro

The nitrergic nerve appears to have a major role in the neuronal regulation of penile erection. Cholinergic innervation has been shown histochemically in penile cavernous tissues, but its functional role is not well understood. This study was aimed at examining the functional properties of the nitrergic nerve and the possible involvement of cholinergic function in the regulation of monkey penile erection in vivo and in vitro. In anesthetized Japanese monkeys, electrical stimulation of the cavernous nerve caused a frequency-dependent increase in intracavernous pressure and penile erection, and atropine enhanced the pressure response. Intravenous injections of N(G)-nitro-L-arginine (L-NA) markedly inhibited the stimulation-induced pressure increase and the erectile response, and L-arginine partially restored the pressure response. In some monkeys, the intracavernous pressure increase caused by nerve stimulation was reversed by treatment with L-NA; however, L-arginine restored the pressor response. In addition, hexamethonium suppressed the pressure increase that resulted from the nerve stimulation. In corpus cavernosum isolated from monkeys, transmural electrical stimulation elicited frequency-dependent relaxation. The relaxation was attenuated by physostigmine, and was potentiated by atropine. Relaxation was markedly inhibited by treatment with L-NA. It appears that nitric oxide (NO) released from inhibitory nerves, even at low frequencies, has a pivotal role in the initiation and maintenance of intracavernous pressure increase and penile erection in monkeys. Prejunctional muscarinic receptors in nitrergic nerves are expected to participate in the impairment of NO release. Nitrergic nerves responsible for penile erection may originate from ganglia close to the corpus cavernosum.

Vasodilating beta-adrenoceptor blockers as cardiovascular therapeutics

beta-Adrenoceptor blocking agents (beta-blockers) have been established as therapeutics for treatment of patients with hypertension, ischemic heart diseases, chronic heart failure, arrhythmias, and glaucoma. However, their clinical use is limited because some patients are adversely affected by their side effects. The discovery of cardioselective (beta(1)-selective) blockers has overcome some of the problems. Current retrospective studies have revealed that vasodilating beta-blockers (so-called beta-blockers of the third generation) have advantages over the conventional type of beta-blockers in terms of minimizing the adverse effects and improving the disease-derived dysfunction, thus enhancing the quality of life variables. Some of the possible advantages include improvement of insulin resistance, decrease in low-density lipoprotein cholesterol in association with increase in high-density lipoprotein cholesterol, attenuation of bronchial asthma attack and respiratory dysfunction, alleviation of coronary vasospasm provocation, peripheral circulatory disturbances, and erectile dysfunction, and better patient compliance. Release of nitric oxide, antioxidant action, beta(2)-adrenoceptor activation, Ca(2+) entry blockade, and other mechanisms underlying the vasodilating action may be responsible for the beneficial therapeutic effects of these agents.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

[Nitroxidergic (nitrergic) nerve and erectile dysfunction]

In vascular tissues including the corpus cavernosum, the organ function is reciprocally regulated by noradrenergic and non-adrenergic, non-cholinergic (NANC) nerves. NANC nerves innervating the corpus cavernosum is thought to be nitroxidergic (nitrergic) nerves which liberate nitric oxide (NO) produced by neuronal NO synthase, and liberated NO activates soluble guanylate cyclase (sGC) in cavernous smooth muscle cells. Intracellular increase in cyclic (c) GMP by activation of sGC dilates cavernous smooth muscle and then induces penile erection. Nitroxidergic (nitrergic) vasodilator nerves also innervate cavernous arteries and veins which regulate the blood volume in the corpus cavernosum. The order of potency of nitroxidergic nerve functions in these tissues (cavernosum > artery >> vein) may be suitable for producing the erection. Therefore, obstruction of the arteries and impairment of nitroxidergic (nitrergic) nerve function are speculated to be one of the causes for erectile dysfunction (ED). On the other hand, NO derived from the cavernous endothelium may partly contribute to erectile function. Sildenafil (Viagra) is one of the potent therapeutics for ED. The agent is a selective phosphodiesterase type 5 (PDE-V) inhibitor that inhibits degradation of cGMP elevated by NO mainly derived from the nerves. To develop more selective and safer therapeutics for ED, further systematic investigations are required.

Effects of calcium antagonists on the nitrergic nerve function in canine corpus cavernosum

Effects of calcium antagonists on nitrergic nerve function were examined in the isolated canine corpus cavernosum. In the cavernous strips precontracted with phenylephrine, transmural electrical stimulation elicited frequency-dependent (2 - 5 Hz) relaxations that were abolished by N(G)-nitro-L-arginine (10(-5) M), a nitric oxide (NO) synthase inhibitor; 1H[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ, 10(-6) M), a soluble guanylate cyclase inhibitor; and tetrodotoxin (3 x 10(-7) M). The relaxations were not affected by treatment with nifedipine or nicardipine (10(-8) - 10(-6) M), L-type specific calcium channel inhibitors, but were significantly inhibited by amlodipine or cilnidipine, inhibitors of L- plus N-type calcium channels, in a concentration-related manner (10(-7) - 10(-6) M). All of the inhibitors used did not affect the relaxations induced by exogenous NO (acidifed NaNO2). These findings suggest that N-type, but not L-type, calcium channels are responsible for increasing cytosolic free calcium, a prerequisite for the synthesis of NO, in the nitrergic dilator nerves innervating the corpus cavernosum.

Pharmacological profile of T-1032, a novel specific phosphodiesterase type 5 inhibitor, in isolated rat aorta and rabbit corpus cavernosum

This study was designed to examine the pharmacological properties of T-1032 (methyl-2-(4-aminophenyl)-1,2-dihydro-1-oxo-7-(2-pyridinylmethoxy)-4-(3,4,5-trimethoxyphenyl)-3-isoquinoline carboxylate sulfate), a novel phosphodiesterase type 5 inhibitor, in isolated rat aorta and rabbit corpus cavernosum. T-1032 (3x10(-11) to 3x10(-7) M) caused an endothelium-dependent relaxation in the isolated rat aorta precontracted with phenylephrine, and the relaxation was accompanied by an increase in cGMP but not cAMP levels. The T-1032-induced relaxation was attenuated by N(G)-nitro-L-arginine methyl ester (L-NAME) (10(-3) M), a nitric oxide (NO) synthase inhibitor, or 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ) (10(-5) M), a guanylyl cyclase inhibitor. T-1032 (10(-9), 10(-8) M) produced a potentiation of the relaxation induced by sodium nitroprusside, but not of the relaxation induced by isoproterenol. In the isolated rabbit corpus cavernosum precontracted with phenylephrine, the electrical field stimulation-induced relaxation was attenuated by treatment with tetrodotoxin (10(-6) M) as well as L-NAME (10(-4) M). The L-NAME-inhibited relaxation was restored by treatment with L-arginine (5x10(-4) M). T-1032 (10(-9) to 10(-6) M) and sildenafil (10(-9) to 10(-6) M) produced a potentiation of the electrical field stimulation-induced relaxation as well as a decrease in basal tension in a concentration-dependent manner. It was concluded that T-1032 had potentiating effects on the NO/cGMP signaling pathway in isolated tissues, probably through specific blockade of phosphodiesterase type 5. T-1032 would be a useful compound to examine the physiologic functions of phosphodiesterase type 5 in mammalian tissues.