Evaluation of erectile response by continuous measurement of penile diameter in rats

Intracavernosal Pressure Recording to Evaluate Erectile Function in Rodents

Erectile dysfunction (ED) is defined as the inability to attain or keep an erection of the penis, and this has become a prevalent male sexual disorder. Rodents are employed by many studies to research the physiology/pathology of erectile function. Erectile function in rodents can be evaluated by measuring the intracavernosal pressure (ICP). In practice, ICP can be monitored following electrical stimulation of the cavernous nerves (CNs). The arterial pressure of the carotid artery (the mean arterial pressure) is used as the reference for ICP. Using ICP recording protocols, many key parameters of erectile function can be measured from the ICP response curve. The ICP measurement provides more information than the apomorphine-induced penile erection test, and is cheaper than telemetric monitoring of the corpus spongiosum penis, making this method the most popular one to evaluate erectile function. However, compared to the easily-performed APO-induced erectile function test, successful ICP recordings require attention to detail, practice, and adherence to the operation method. In this work, an introduction to ICP recording in rats is provided to complement the procedure efficiently.

Dual regulation of the ATP-sensitive potassium channel by activation of cGMP-dependent protein kinase

Adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels couple cellular metabolic status to membrane electrical activity. In this study, we performed patch-clamp recordings to investigate how cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG) regulates the function of K(ATP) channels, using both transfected human SH-SY5Y neuroblastoma cells and embryonic kidney (HEK) 293 cells. In intact SH-SY5Y cells, the single-channel currents of Kir6.2/sulfonylurea receptor (SUR) 1 channels, a neuronal-type K(ATP) isoform, were enhanced by zaprinast, a cGMP-specific phosphodiesterase inhibitor; this enhancement was abolished by inhibition of PKG, suggesting a stimulatory role of cGMP/PKG signaling in regulating the function of neuronal K(ATP) channels. Similar effects of cGMP accumulation were confirmed in intact HEK293 cells expressing Kir6.2/SUR1 channels. In contrast, direct application of purified PKG suppressed rather than activated Kir6.2/SUR1 channels in excised, inside-out patches, while tetrameric Kir6.2LRKR368/369/370/371AAAA channels expressed without the SUR subunit were not modulated by zaprinast or purified PKG. Lastly, reconstitution of the soluble guanylyl cyclase/cGMP/PKG signaling pathway by generation of nitric oxide led to Kir6.2/SUR1 channel activation in both cell types. Taken together, here, we report novel findings that PKG exerts dual functional regulation of neuronal K(ATP) channels in a SUR subunit-dependent manner, which may provide new means of therapeutic intervention for manipulating neuronal excitability and/or survival.

Penile erectile responses to cavernous nerve stimulation in rats are not affected by nitrate tolerance

The aim of this study was to elucidate whether penile erection induced by electrical stimulation of the cavernous nerve was affected in male rats with nitrate tolerance. Nitrate tolerance to nitroglycerin was induced by oral administration of isosorbide dinitrate (ISDN) at 1000 mg/kg to rats, once or twice a day for 5 or 6 days. The rats were anesthetized with sodium pentobarbital 18-24 h after the last dosing with ISDN or its vehicle. Penile erection induced by electrical stimulation was monitored by measuring the penile diameter sonomicrometrically. After measurement of the penile erectile response, nitroglycerin (3-300 microg/kg) was intravenously (i.v.) injected into eight rats treated with the vehicle or ISDN to examine its hypotensive effect. In the vehicle-treated rats, the maximal developed penile diameter (D-max) and the duration of penile erection (T50%, period of time from the maximum erection to its 50% decline) produced by electrical stimulation were 509+/-47 microm and 14.2+/-1.7 s, respectively. On the other hand, neither D-max nor T50% in ISDN-treated rats (509+/-36 microm and 13.1+/-1.3 s, respectively) was different from those in the vehicle-treated rats. However, the hypotensive effects of i.v. injected nitroglycerin were significantly attenuated in the ISDN-treated group as compared with the vehicle-treated group. It is concluded that nitrate tolerance fails to influence penile erection induced by cavernous nerve electrical stimulation in rats.

Effects of intracavernous administration of adrenomedullin on erectile function in rats

We have reported that adrenomedullin (AM)-induced vasodilation is at least in part nitric oxide (NO)-cGMP-dependent in the rat. Although it is well known that NO is much involved in the erectile function, it is controversial as to whether AM influences the erectile function. Thus, we examined the effects of AM on intracavernous pressure (ICP) during penile erection. The left carotid artery of rats was cannulated to monitor of mean arterial pressure (MAP). Bipolar electrodes were positioned on the cavernous nerve. The right cavernous body was cannulated with a needle connected to a pressure transducer to monitor ICP. Electrical stimulation (ES) increased ICP in a voltage-dependent manner. Elevation of ICP continued during ES. The intracavernous injection of 0.5 nmol AM significantly potentiated ES-induced increases in both maximal developed ICP/MAP and area under the curve (ICP trace; AUC). Since AM slightly lowered MAP, ICP was normalized by MAP. i.v. administration of N(omega)-nitro-L-arginine, a NO synthase inhibitor, markedly decreased AM/ES-induced ICP elevation. However, in the presence of E-4021, a cGMP-specific phosphodiesterase inhibitor, AM further increased both ICP/MAP and AUC. These results suggest that a NO-cGMP pathway is involved in the regulation of AM-induced rat cavernous vasorelaxation.

General use of animal models for investigation of the physiology of erection

In review, animal models have accounted significantly for the amazing strides made in the field of sexual dysfunction research. Fundamentally, they have offered a unique experimental approach to test many hypotheses regarding sexual function. Since their early use for sexual physiology research, there has been increasing sophistication using animals involving techniques for stimulating and monitoring sexual responses. One specific area that has been advanced is the use of conscious animal models to obtain a better sense of the natural contexts for sexual physiology and to avoid pharmacological interference associated with anesthetics. Another area of interest is the increasing use of simple but valid techniques to record and assess sexual responses. Efforts to develop and evaluate animal models that replicate disorders of sexual function have also been most advantageous. In the future, animal models will remain useful. The expanded applications of animal models include the study of predisposing disease states associated with sexual dysfunction and the study of all aspects of sexual dysfunction, in both male and female subjects. Continued judgment must be applied, understanding the advantages of one or another animal model, to explore questions and provide answers that are most scientifically relevant to the human condition. The promise of advancing therapies in this field indicates the additional prominent role for animals for the purposes of drug development.