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

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Abstract
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, ≤59 years; B, ≥60 years) and further subdivided into two diagnostic categories, diabetic and nondiabetic patients, respectively. In ≈75% of precontracted corporal tissue strips derived from impotent patients (contracted to ≈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 (≈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 EC50for NTG-induced relaxation of precontracted corporal smooth muscle strips from potent patients (≈25 nM) was 0.90 log unit less than that for equivalently contracted corporal smooth muscle strips derived from impotent patients (≈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.Key words: nitroglycerine, corporal smooth muscle, relaxation, desensitization, guanylate cyclase.