Endothelin-1-induced modulation of contractile responses elicited by an alpha 1-adrenergic agonist on human corpus cavernosum smooth muscle.

The goal of these studies was to examine endothelin-1 (ET-1)-induced modulation of contractile responses elicited by the selective alpha 1-adrenergic agonist, phenylephrine (PE), on isolated human corporal tissue strips. Pharmacological studies were conducted on human corporal tissue strips obtained from 22 patients undergoing implantation of penile prostheses for erectile dysfunction. For the purposes of statistical analysis, the patients were stratified into two age groups: A, age < or = 59 y (n = 10) and B, age > or = 60 y (n = 12). The patients were further sub-divided into two diagnostic categories, diabetics (DM, n = 9) and nondiabetics (ND, n = 13). Cumulative concentration-response curves (CRCs) were constructed to the alpha 1-adrenergic agonist, PE, prior to constructing a CRC to a single mixture of PE and ET-1 on the same tissue. A previously described fixed molar ratio (FMR) protocol was used to generate CRCs to mixtures of PE and ET-1. In all cases, for the PE:ET-1 FMRs of 90:10, 80:20 and 70:30, the partial substitution of PE with ET-1 resulted in an approx 3-fold leftward shift in the EC50 of the PE alone CRC with an approx 4% concomitant increase in Emax and a decrease in the slope factor value. There were no significant age- or disease-related differences in any of the logistic parameter estimates that describe the FMR CRC, indicating that there are no detectable age- or disease-related alterations in ET-1-induced amplification of alpha 1-adrenergic-mediated contractions in these studies. In addition, the location of the FMR CRC was precisely predicted by the theoretical CRC for simple additivity of agonist effects. In conclusion, since relatively small increases in ET-1 concentrations were associated with significant increases in alpha 1-adrenergic-mediated contractile responses, these data provide further testimony to the importance of ET-1 in modulating corporal smooth muscle tone, and moreover, establish a conceptual framework for understanding the mechanism of its action(s).

Characterization of nitroglycerine-induced relaxation in human corpus cavernosum smooth muscle: implications to erectile physiology and dysfunction.

The importance of the nitric oxide--guanylate cyclase--cGMP system in modulating corporal smooth muscle tone and penile erection has been amply demonstrated. The goal of these studies was to evaluate the possibility that age- or disease-related alterations in human corporal smooth muscle responsivity to activation of this pathway might play a role in the etiology of erectile dysfunction. Thus, we utilized a previously described heuristic model to assess the kinetic and steady-state characteristics of relaxation of precontracted isolated corporal tissue strips elicited by nitroglycerine (NTG). Studies were conducted on corporal tissue strips excised from 26 patients with organic erectile dysfunction, and 7 patients with documented erections. For the purposes of statistical analysis the impotent patient population was stratified into two age groups (A, < or = 59 years; B, > or = 60 years) and further subdivided into two diagnostic categories, diabetic and nondiabetic patients, respectively. In approximately 75% of precontracted corporal tissue strips derived from impotent patients (contracted to approximately 75% of maximum with phenylephrine), the NTG-induced response was biphasic, consisting of a rapid relaxation response that reached steady state before onset of a more slowly developing regaining of tension, termed the desensitization response. In contrast, a biphasic response was observed much less frequently (approximately 30%) in corporal tissue strips derived from a potent patient population (p < 0.0001). Statistical analysis revealed significant heterogeneity among corporal tissue strips derived from patients with organic erectile dysfunction, with respect to both the kinetic and steady-state characteristics of the NTG-induced relaxation and desensitization responses. In particular, the maximal rate constant for both NTG-induced relaxation (krelmax; p < 0.01) and desensitization (kdes; p < 0.03) responses was significantly greater in corporal tissue strips excised from diabetic than nondiabetic patients. Furthermore, the EC50 for NTG-induced relaxation of precontracted corporal smooth muscle strips from potent patients (approximately 25 nM) was 0.90 log unit less than that for equivalently contracted corporal smooth muscle strips derived from impotent patients (approximately 180 nM; p < 0.03). Such observations suggest that alterations in corporal smooth muscle responsivity to activation of the guanylate cyclase--cGMP pathway, per se, may be a characteristic of organic erectile dysfunction. In the absence of compensatory changes in other vasodilatory mechanisms, this may contribute to incomplete corporal smooth muscle relaxation and the etiology of erectile dysfunction in some patients.

Gap junctions modulate tissue contractility and alpha 1 adrenergic agonist efficacy in isolated rat aorta.

Immunocytochemical analysis, using antibodies directed against connexin43, revealed abundant gap junctions between smooth muscle cells in intact aorta from Fischer 344 rats. Therefore, the authors evaluated the potential contribution of these intercellular junctions to contractile responses elicited by alpha 1 adrenergic receptor activation in rat aortic rings. Preincubation with the selective junctional uncoupling agent heptanol (200 microM) diminished the magnitude of contractions induced by the low-efficacy partial agonist oxymetazoline (1-3 microM) by 50.6 +/- 4.5% (P < .01; n = 16 rings from 16 rats) but had no effect on equivalent contractions induced by the high-efficacy agonist phenylephrine (0.1 microM; n = 16 rings from 16 animals). Reduced phenylephrine contractility was observed at higher heptanol concentrations (500 microM). However, neither 200 nor 500 microM heptanol altered the magnitude of contractions elicited by 60 mM KCl, indicating that tissue contractility per se was unaffected by heptanol. In calcium-free solution, the magnitude of the phasic contraction induced by phenylephrine was three-fold greater than the magnitude of the oxymetazoline-induced phasic contraction (P < .001) but the phasic responses to both agonists were unaffected by the same heptanol concentrations that significantly diminished their steady-state responses. Because heptanol, at the concentrations used, has selective pharmacological actions on gap junctions, these studies provide additional support for a role of gap junctions in the maintenance and modulation of vasomotor tone. In rat aorta, junctional transfer of alpha 1 adrenergic-receptor activated second-messenger molecules appears to be an important modulator of tissue contractility and agonist efficacy.

Intercellular communication through gap junctions: a potential role in pharmacomechanical coupling and syncytial tissue contraction in vascular smooth muscle isolated from the human corpus cavernosum.

Kinetic and steady-state protocols were used to examine the effects of disruption of intercellular communication with heptanol, on contractile responses elicited by activation of the alpha 1-adrenergic receptor in human corporal vascular smooth muscle. For the steady-state studies, strips of corporal tissue from 19 patients were submaximally precontracted with phenylephrine (PE) and then relaxed by the cumulative addition of heptanol. Heptanol completely and reversibly relaxed all tissues studied in a concentration-dependent manner. The heptanol concentration response data were then computer fit to the general logistic equation to obtain pEC50 (negative logarithm of the concentration that elicits one-half of the maximal effect) and slope factor values, with Emax (maximal relaxation) set to 100%. The mean pEC50 and slope factor values, respectively, were 2.86 +/- 0.04 and 1.86 +/- 0.17. Furthermore, kinetic studies on corporal tissues from a subset of the patient population (11 patients) revealed that preincubation of tissues with 2 mM heptanol caused a significant decrease in both the rate and magnitude of PE-induced contractions in all tissues studied, without affecting the rate constant for onset of contraction (k(obs)). The present results on intact tissue extend our previous observations on cultured corporal cells, and support the supposition that intercellular communication through gap junctions may play an important role in the initiation, maintenance and modulation of alpha 1-adrenergic contractions in human vascular smooth muscle.