Magnus Fovaeus

Muscarinic Receptor Stimulation of Phosphoinositide Hydrolysis in the Human Isolated Urinary Bladder

The stimulatory action of carbachol and acetylcholine (ACh) on phosphoinositide turnover, as well as their contractile effects, were investigated in human isolated detrusor muscle. Carbachol, and ACh in combination with 10(-7) M physostigmine, induced increases in phosphoinositide turnover. However, at all the concentrations tested, carbachol was more effective than ACh (plus physostigmine), and at the highest concentration used (10(-4) M), the difference was significant (p less than 0.05). Also in a Ca(2+)-free medium containing the chelator EGTA (10(-4) M), both agonists (10(-4) M) induced small but distinct increases in phosphoinositide breakdown. Carbachol and ACh contracted the detrusor preparations concentration-dependently, and the responses were almost identical when ACh was combined with 10(-7) M physostigmine. In Ca(2+)-free medium the agonists elicited a moderate but concentration-dependent contractile response at high concentrations. The results show that muscarinic receptor agonists stimulate phosphoinositide turnover in the human bladder. Possibly, this effect is coupled to multiple muscarinic receptor subtypes. More studies are required to elucidate to what extent phosphoinositide breakdown participates in the contractile activation of this tissue.

Effects of some calcium channel blockers on isolated human penile erectile tissues.

The effects of the calcium channel blockers (CCBs) verapamil, nifedipine and diltiazem on contractile activation of isolated human penile erectile tissues were investigated. Specimens of the corpus spongiosum (CS) and corpora cavernosa (CC) were obtained from men with a history of normal penile erection undergoing cystourethrectomy because of bladder malignancy. Preparations were mounted in organ baths and isometric tension was recorded. Deprivation of extracellular calcium abolished electrically induced contractions in both CS and CC preparations within 15 min.; norepinephrine (NE)-induced contractions were reduced by 90% (CS) and 83% (CC) after 30 min. All the CCBs reduced electrically induced contractions concentration-dependently, nifedipine being the most potent agent. Contractions induced by exogenous NE were depressed by about 50%, whereas high K+ (124 mM) induced responses were abolished. It is concluded that contraction in penile erectile tissues is mediated mainly by neuronally released NE stimulating postjunctional alpha-adrenoceptors. The contraction is highly dependent on extracellular calcium and can partly be inhibited by CCBs. It cannot be excluded that some CCBs injected intracavernosally may be useful for diagnosis and even treatment of erectile dysfunction. However, calcium channel blockade may not be as effective as a therapeutic principle as blockade of alpha-adrenoceptors.

Effects of Cromakalim (BRL 34915) and Pinacidil on Normal and Hypertrophied Rat Detrusor in Vitro

Normal and hypertrophied rat detrusor were investigated in vitro with regard to effects of the K(+)-channel openers pinacidil and cromakalim. Both drugs abolished spontaneous contractile activity and induced a relaxation of normal and hypertrophied detrusor preparations. In both types of preparation, contractions elicited by K+, carbachol or electrical field stimulation were depressed in the presence of the K(+)-channel openers. Responses induced by K+ or electrical stimulation were more reduced in the hypertrophied than in the normal detrusor. Both K(+)-channel openers increased the efflux of 86Rb+ in a concentration-dependent way and this increase was similar in normal and hypertrophied detrusor. If applicable to man, this data suggest that K(+)-channel openers may be effective in the treatment of bladder instability secondary to outflow obstruction.

K(+)-channel openers for relaxation of isolated penile erectile tissue from rabbit.

The effects of the K(+)-channel openers (KCOs) cromakalim (BRL 34915) and pinacidil were investigated and compared with those of papaverine on isolated corpus cavernosum from rabbit. Preparations were mounted in organ baths and isometric tension was recorded. Spontaneous contractile activity was effectively abolished by the KCOs tested, cromakalim being the most potent of them. The KCOs concentration-dependently and effectively depressed electrically induced contractions and also contractions induced by exogenously applied noradrenaline and by low (less than or equal to 20 mM) concentrations of K+. Cromakalim was three to four times more potent than pinacidil. Pinacidil and cromakalim were shown to cause increases in the efflux of 86Rb from preloaded cavernous tissue. Papaverine also effectively depressed spontaneous contractile activity, and contractions evoked by electrical stimulation and noradrenaline. It had a potency 19 to 36 times lower than that of cromakalim. However, papaverine did not increase 86Rb efflux from preloaded tissue. The results show that cromakalim and pinacidil effectively relax penile erectile tissue, probably by the opening of K(+)-channels and subsequent hyperpolarization. Further investigations on human material seems motivated in order to elucidate if the principle of K(+)-channel opening offers any therapeutic advantages to other drugs in the diagnosis and treatment of penile erectile dysfunction.

The Action of Pinacidil in the Isolated Human Bladder

The need for effective symptomatic treatment of patients with detrusor hyperactivity is widely recognized. In search of new principles of decreasing bladder contraction we have studied the effects of pinacidil on the isolated human bladder. Pinacidil is a recently developed antihypertensive agent classified as a K+ channel opener, and is believed to depress smooth muscle activity by this action. Pinacidil concentration-dependently depressed contractions elicited by carbachol, low concentrations of K+ (less than 60 mM) and electrical stimulation. In addition it caused a concentration-related increase in the efflux of 86Rb from preloaded detrusor cells. The effects on 86Rb efflux could be inhibited by tetraethylammonium chloride and procaine, but not by apamin, agents known to block K+-channels. The results support the view that part of the pinacidil effect on the human bladder is caused by an opening of K+-channels, efflux of K+ and subsequent hyperpolarization of the detrusor cells. Clinical testing of this new therapeutic principle for treatment of bladder hyperactivity seems justified.

Calcium channel blockade and contractile responses in the isolated human vas deferens.

The effects of removal of extracellular calcium and of the calcium channel blockers nifedipine, verapamil and diltiazem were studied on contractions induced by electrical field stimulation and high K+-solution in isolated preparations of the human vas deferens. Electrically induced contractions were blocked by tetrodotoxin and alpha-adrenoceptor blockade. They were abolished in calcium-deficient medium, and suppressed by the calcium channel blockers in the order of potency nifedipine greater than verapamil greater than diltiazem. The maximum blocking effect of nifedipine was approximately 40%. All the blockers practically abolished K+-induced contractions. It is concluded that even if the contractile response of the human vas deferens to electrical stimulation is dependent on extracellular calcium, calcium channel blockers seem to have only a limited effect on this contraction and their capability of impairing the function of the vas deferens in patients is questioned.

Immunoreactive Arginine Vasopressin (AVP) and Effects of Avp in the Human Vas Deferens

Immunoreactive (IR) arginine vasopressin (AVP) was found to occur in the epididymal part of the human vas deferens. Segments from nine different subjects all contained IR-AVP in concentrations ranging from 37 to 717 fmol/gm. wet weight, concentrations severalfold higher than those normally found in the circulation. IR-AVP was shown by high performance liquid chromatography to elute in the same position as synthetic AVP. AVP added to isolated preparations of the human vas deferens induced concentration-related repetitive phasic contractions without significant changes of baseline tension. These contractions seemed to be mediated via stimulation of vasopressin V1-receptors and were abolished in the presence of vasopressin antagonists. Contractions induced by electrical field stimulation were frequency-dependent and sensitive to tetrodotoxin and prazosin. They were not affected by the vasopressin antagonists used. AVP increased the response to electrical field stimulation and this effect was inhibited by vasopressin antagonists. The results suggest either that circulating AVP is taken up and accumulated by the human vas deferens, and/or that AVP is synthesized locally. They do not suggest co-release of AVP and noradrenaline from nerve endings. The physiological role of the AVP occurring in the human vas deferens remains to be established.

Characterization of Immunoreactive Arginine Vasopressin (AVP) in and Effects of AVP on Isolated Human Penile Erectile Tissues

Arginine vasopressin (AVP), which has been shown to have a marked contractant effect on human penile erectile tissues, particularly the corpus spongiosum (CS), was demonstrated to occur in these tissues. Out of nine CS samples, eight contained vasopressin-like immunoreactivity in concentrations ranging from 3.5 to 107.2 fmol./gm. wet weight. These amounts suggest that the hormone is taken up and/or synthesized locally. The effects of four different vasopressin antagonists were characterized on human CS strips, and they were found to effectively inhibit the contractile response to exogenous AVP. However, none of these antagonists had any effects on electrically induced contractions in penile erectile tissues. These results do not favour the view that AVP is released on electrical stimulation in amounts that contract the erectile tissues. Whether or not the peptide is involved in the mechanisms controlling penile erection remains unclear.