All NOS subtypes produce NO, but each may play a different biologic role in various tissues. nNOS and eNOS are considered constitutive forms because they share biochemical features: They are calcium-dependent, they require calmodulin and reduced nicotinamide adenine dinucleotide phosphate for catalytic activity, and they are competitively inhibited by arginine derivatives. nNOS is involved in the regulation of neurotransmission, and eNOS is involved in the regulation of blood flow.
But if the blood flow is weak here, it is highly likely that it is also weak in arteries supplying the heart, raising the risk of a heart attack. In fact, some studies suggest that women with heart disease may also suffer sexual dysfunction: the clitoris, like the penis, is a vascular organ, and also relies on healthy blood flow for successful orgasms.

Under normal circumstances, when a man is sexually stimulated, his brain sends a message down the spinal cord and into the nerves of the penis. The nerve endings in the penis release chemical messengers, called neurotransmitters, that signal the corpora cavernosa (the two spongy rods of tissue that span the length of the penis) to relax and fill with blood. As they expand, the corpora cavernosa close off other veins that would normally drain blood from the penis. As the penis becomes engorged with blood, it enlarges and stiffens, causing an erection. Problems with blood vessels, nerves, or tissues of the penis can interfere with an erection.
Physicians make a diagnosis of erectile dysfunction in men who complain of troubles having a hard enough erection or a hard erection that does not last long enough. It is important as you talk with your doctor that you be candid in terms of when your troubles started, how bothersome your erectile dysfunction is, how severe it is, and discuss all your medical conditions along with all prescribed and nonprescribed medications that you are taking. Your doctor will ask several questions to determine if your symptoms are suggestive of erectile dysfunction and to assess its severity and possible causes. Your doctor will try to get information to answer the following questions:
Occasional successful sexual function and early morning erections do not preclude the possibility of endocrine dysfunction. Since abnormally low levels of testosterone frequently are the primary cause of impotence, it is recommended that determination of the blood level of testosterone be an integral part of the total evaluation of the impotent patient.
For some patients with an established diagnosis of testicular failure (hypogonadism), androgen replacement therapy may sometimes be effective in improving erectile function. A trial of androgen replacement may be worthwhile in men with low serum testosterone levels if there are no other contraindications. In contrast, for men who have normal testosterone levels, androgen therapy is inappropriate and may carry significant health risks, especially in the situation of unrecognized prostate cancer. If androgen therapy is indicated, it should be given in the form of intramuscular injections of testosterone enanthate or cypionate. Oral androgens, as currently available, are not indicated. For men with hyperprolactinemia, bromocriptine therapy often is effective in normalizing the prolactin level and improving sexual function. A wide variety of other substances taken either orally or topically have been suggested to be effective in treating erectile dysfunction. Most of these have not been subjected to rigorous clinical studies and are not approved for this use by the Food and Drug Administration (FDA). Their use should therefore be discouraged until further evidence in support of their efficacy and indicative of their safety is available.
Psychological causes include depression, stress and anxiety. Men sometimes worry about getting a new sexual partner pregnant so do make sure you mention contraception. If the relationship is relatively new your partner might be nervous about ‘performing’. If they don’t manage to get an erection, this can add to the tension and pressure the next time and further compound the issue.
In the majority of patients the impotence is organic, though not endocrinologic, and there is no easily remedied cause. These patients require physiologic testing and urologic consultation for specific diagnosis. Likely causes of impotence in this group include vascular and neurologic diseases. These patients are candidates for penile prostheses or, in special cases, for revascularization. Patients interested in surgical approaches should be referred for further testing. There is little to be gained by continuing the work-up of patients who prefer not to have an operation.
Lifestyle choices that impair blood circulation can contribute to ED. Smoking, excessive drinking, and drug abuse may damage the blood vessels and reduce blood flow to the penis. Smoking makes men with atherosclerosis particularly vulnerable to ED. Being overweight and getting too little exercise also contribute to ED.  Studies indicate that men who exercise regularly have a lower risk of ED.
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