In vivo electrochemical measurement of nitric oxide in corpus cavernosum penis Manuel Mas *, Ana Escrig, Jose Luis Gonzalez-Mora Department of Physiology and CESEX, School of Medicine, University of La Laguna, 38071 Tenerife, Spain Abstract A wealth of pharmacological studies suggest that nitric oxide (NO) generated in the corpus cavernosum is a main molecular mediator of penile erection. However, the physiological levels of NO in the corpora and their possible changes during penile erection have remained unknown for want of suitable methodologies. We have adapted a voltammetric procedure, derived from Malinski’s method, for assessing NO levels in the penis in vivo. Differential normal pulse voltammetry with carbon fiber electrodes (30 mm) coated with a polymeric porphyrin and Nafion was used to measure the NO oxidation current in the corpora cavernosa of urethane- anesthetized rats. The intracavernous pressure was monitored simultaneously. A NO oxidation peak was consistently detected at approximately 650 mV both in NO solutions and in the corpora in vivo. The changes in the NO signals observed in vitro were consistent with the concentration values measured by chemiluminescence. The NO signal recorded in vivo increased following cavernous nerve stimulation and was greatly decreased by intracavernous injections of several inhibitors of the neuronal and endothelial NO synthase isoenzymes. Such results agree with our previous studies using this methodology and substantiate further its validity for monitoring the physiological changes in NO levels in the penis. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nitric oxide; Voltammetry; Penile erection; Nitric oxide synthase inhibitors; Chemiluminescence 1. Introduction Penile erection results from the enlargement and rigidity of the corpora cavernosa penis by accumulation of blood inside them. The cavernous tissue consists of a mesh of trabeculae */bundles of smooth muscle cells, fibroblasts, collagen, elastin and nerve endings */form- ing the walls of the endothelium-lined sinusoids or lacunae. The key event for the development and maintenance of penile erection is the relaxation of the smooth muscle cells in the cavernous trabeculae and arterioles. That allows the filling of the sinusoids with arterial blood thereby increasing the corporeal (i.e. penile) volume. The veins become compressed against the sturdy tunica albuginea encasing the corpora and the ensuing reduc- tion in venous outflow facilitates penile rigidity. Con- versely, the contraction of the trabecular and arteriolar smooth muscle cells prevents or terminates erection (for review see Andersson and Wagner, 1995). The ca vernous smooth muscle tone is physiologically regulated by a complex interplay of messenger molecules arising from paracrine (endothelium), autocrine, endocrine and, especially important, neural sources (for review see Moreland et al., 2001). Thus, the corpora cavernosa are innervated by both proerectile parasympathetic fibers contained in the pelvic nerves (nervii erigentes) entering the corpora in the cavernous nerves, and antierectile sympathetic (noradrenergic) nerves from the hypogastric plexus. It is currently thought that the main erectogenic signal used by the cavernous nerves is nitric oxide (NO) (for reviews see Lugg et al., 1995; Burnett, 1997), although other potentially important neurotransmitters cannot be excluded. This messenger gas is formed from the amino acid L-arginine by a family of three NO synthase (NOS) enzymes encoded by different genes. The two ‘constitu- tive’, Ca 2 dependent isoforms, first discovered in neurons (nNOS) and endothelial cells (eNOS), are both well expressed in the penis, although the ca vernous nerves are currently regarded as the main source of NO in the corpora (Andersson and Wagner, 1995). A third, * Corresponding author. Tel.:  /34-922-655847; fax:  /34-922- 319397 E-mail address: mmas@ull.es (M. Mas). Journal of Neuroscience Methods 119 (2002) 143 /150 www.elsevier.com/locate/jneumeth 0165-0270/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0165-0270(02)00173-5