0022-534 7/92/14 7 4-1124$03.00/0 THE JOURNAL OF UROLOGY Copyright© 1992 by AMERICAN UROLOGICAL ASSOCIATION, INC.
Vol. 147, 1124-1128, April 1992
Printed in U.S.A.
INTRACAVERNOUS PRESSURE AS AN EXPERIMENTAL INDEX IN A RAT MODEL FOR THE EVALUATION OF PENILE ERECTION KUANG-KUO CHEN, JULIE Y. H. CHAN, LUKE S. CHANG, MING-TSUN CHEN SAMUEL H. H. CHAN*
AND
From the Division of Urology, Department of Surgery, and Institute of Pharmacology, National Yang-Ming Medical College, and Department of Medical Research, Veterans General Hospital-Taipei, Taipei, Taiwan, Republic of China
ABSTRACT
This report communicates our attempt to design a small animal model for the evaluation of penile erection, based on the pharmacological responses of cavernous tissues in the rat that resemble those of human subjects. Male adult Sprague-Dawley rats anesthetized with pentobarbital sodium were used in conjunction with papaverine and prostaglandin El, two vasoactive drugs most commonly used in clinical management of impotence. Intracavernous administration of papaverine (0.05, 0.1, 0.02, 0.4 or 0.8 mg.) induced a progressive increase in intracavernous pressure that peaked at 0.4 mg. This effect was associated with visible penile erection that became conspicuous when accompanied by additional bursts of transient intracavernous pressure fluctuations. The duration of papaverine-induced increase in intracavernous pressure was significantly shortened by clonidine (15 µg., intracavernous). Injection of prostaglandin El (1, 2 or 4 µg.) into the corpus cavernosum also elicited an elevation in intracavernous pressure, but the responses exhibited acute tachyphylaxis. By manifesting a response to papaverine and prostaglandin El that is similar to that in human, we conclude that the intracavernous pressure in the rat may represent a suitable index for the evaluation of penile erection in small laboratory animals. KEY WORDS:
penile erection, papaverine, prostaglandin, clonidine, rats
Whereas male impotence presents itself as an important clinical issue, investigations into its mechanisms only became more extensive during the past decade. 1- 5 At the end-organ level, it is now well known that activation of sacral parasympathetic outflow or stimulation of cavernous nerve induces erection, and sympathetic vasoconstrictor neurons in the pudenda! and hypogastric nerves elicit detumescence of the penis. 2 • 6 The participation of neurotransmitters other than acetylcholine and norepinephrine, and some neuropeptides (vasoactive intestinal peptide, substance P), during the erectile process is also suggested. 6- 8 The role of the central nervous system in penile erection, however, remains relatively unexplored. Administration of vascular smooth muscle relaxants, for example papaverine or prostaglandin El, or a-adrenoceptor antagonists, such as phentolamine, results in penile erection because of an increase in arterial inflow of blood, distension of sinusoids and possible restriction of venous outflow. 9 • 10 Thus, intracavernous injection of vasoactive drugs offers impotent patients another form of therapeutic management, 9- 11 and allows one of the tests for differential diagnosis between vasculogenic and other etiologic forms of impotence. 9 Papaverine and prostaglandin El are also important tools in the assessment of pharmacological response of penile erectile tissues under experimental conditions. 12- 14 Many physiological studies on penile erection were carried out in larger animals, including monkeys, dogs, rabbits and cats. 1 • 3• 4 • 15 • 16 The large volume of tissues and prominent peripheral nervous and blood vessel supplies to the penis in these animals provide convenient access for pharmacological, neurological and hemodynamic evaluations. However, it is becoming financially prohibitive to obtain and maintain monkeys, dogs and cats for experimental purposes. Small laboratory animals such as rats, on the other hand, offer the advantage of Accepted for publication October 23, 1991. * Requests for reprints: Institute of Pharmacology, National YangMing Medical College, Shih-Pai, Taipei 11221, Taiwan, Republic of China.
easy and much less expensive supply of strain-specific and agematched experimental animals for the study of penile erection. Furthermore, the male rat shows many similarities with the behavioral and endocrine aspects of sexual activity in man.17 Therefore, the male rat may be a satisfactory and useful animal model for the investigation of penile erection. We evaluated the feasibility of this model, using directly measured intracavernous pressure in the rat as the experimental index for penile erection. We based our conclusion on the recorded pharmacological responses of rodent penile tissues to papaverine and prostaglandin El, two vasoactive drugs most commonly used in clinical management and diagnosis of impotence. 9- 11 • 18 MATERIALS AND METHODS
Male, adult Sprague-Dawley rats (200 to 300 gm.) anesthetized with pentobarbital sodium (50 mg./kg., i.p., with 10 mg./ kg./hour i.v. supplements) were used. The animal was placed on a heating pad to maintain its body temperature at 37C. The trachea was intubated for better maintenance of airway patency. The femoral artery and vein on one side were cannulated for measurement of arterial pressure and administration of drugs. The former was achieved via a pressure transducer (Gould 23ID) and a Gould pressure processor preamplifier that registers both pulsatile and mean arterial pressures. Heart rate was measured by a tachometer preamplifier (Gould) triggered by the arterial pulses. The skin overlying the penis was incised, and the prepuce was degloved to fully expose both corpora cavernosa. A 26gauge needle filled with saline and connected to a Gould 23ID pressure transducer was carefully inserted into the corpus cavernosum on one side to measure the intracavernous pressure. lntracavernous pressure was monitored alongside pulsatile and mean arterial pressure, and heart rate during the experiment. The effect on the intracavernous pressure of two vasoactive drugs most commonly used in clinical management of impotence, papaverine hydrochloride (U-Liang Pharmaceutical, Taiwan) and prostaglandin El (Ono Pharmaceutical, Japan) was used to evaluate the feasibility of our rat model. Drugs
11 ?,4
1125
RAT MODEL FOR PENILE ERECTION
were given intracavernously via the intracavernous pressure recording needle, at a volume of 0.05 or 0.1 ml. Papaverine (0.05, 0.1, 0.2, 0.4 or 0.8 mg.) or prostaglandin El (1, 2 or 4r µg.) was used to observe the degree and duration of induced changes in intracavernous pressure. Although drugs were given consecutively, a second injection was always delivered only after the intracavernous pressure has returned to baseline level. We also studied the possible detumescent action of clonidine (Jung-Shin Pharmaceutical, Taiwan). 19 The imidazoline compound was injected intracavernously (7.5 or 15 µg.) either subsequent to, or in combination with, papaverine (0.2, 0.4 or 0.8 mg.). Intracavernous injection of saline, at 0.05 and 0.1 ml., was routinely executed at the beginning of the experiment to ascertain that the introduction of fluid directly into the corpus cavernosum may not be a confounding factor. We also monitored the hemodynamic parameters to confirm that the action ofpapaverine, prostaglandin El or clonidine was exerted locally at the cavernous tissue rather than systemically. At the end of each experiment, a small amount of black ink was injected intracavernously to verify the position of the recording needle. The oblique antero-posterior and transverse dimensions of each corpus cavernosum were also measured under a dissecting microscope. All data were presented as mean ± S.E. Differences between treatment groups were evaluated by ANOV A, followed by the Dunnett test for post hoc comparison of means against the control. Values were considered significant at p
Vol. 147, 1124-1128, April 1992
Printed in U.S.A.
INTRACAVERNOUS PRESSURE AS AN EXPERIMENTAL INDEX IN A RAT MODEL FOR THE EVALUATION OF PENILE ERECTION KUANG-KUO CHEN, JULIE Y. H. CHAN, LUKE S. CHANG, MING-TSUN CHEN SAMUEL H. H. CHAN*
AND
From the Division of Urology, Department of Surgery, and Institute of Pharmacology, National Yang-Ming Medical College, and Department of Medical Research, Veterans General Hospital-Taipei, Taipei, Taiwan, Republic of China
ABSTRACT
This report communicates our attempt to design a small animal model for the evaluation of penile erection, based on the pharmacological responses of cavernous tissues in the rat that resemble those of human subjects. Male adult Sprague-Dawley rats anesthetized with pentobarbital sodium were used in conjunction with papaverine and prostaglandin El, two vasoactive drugs most commonly used in clinical management of impotence. Intracavernous administration of papaverine (0.05, 0.1, 0.02, 0.4 or 0.8 mg.) induced a progressive increase in intracavernous pressure that peaked at 0.4 mg. This effect was associated with visible penile erection that became conspicuous when accompanied by additional bursts of transient intracavernous pressure fluctuations. The duration of papaverine-induced increase in intracavernous pressure was significantly shortened by clonidine (15 µg., intracavernous). Injection of prostaglandin El (1, 2 or 4 µg.) into the corpus cavernosum also elicited an elevation in intracavernous pressure, but the responses exhibited acute tachyphylaxis. By manifesting a response to papaverine and prostaglandin El that is similar to that in human, we conclude that the intracavernous pressure in the rat may represent a suitable index for the evaluation of penile erection in small laboratory animals. KEY WORDS:
penile erection, papaverine, prostaglandin, clonidine, rats
Whereas male impotence presents itself as an important clinical issue, investigations into its mechanisms only became more extensive during the past decade. 1- 5 At the end-organ level, it is now well known that activation of sacral parasympathetic outflow or stimulation of cavernous nerve induces erection, and sympathetic vasoconstrictor neurons in the pudenda! and hypogastric nerves elicit detumescence of the penis. 2 • 6 The participation of neurotransmitters other than acetylcholine and norepinephrine, and some neuropeptides (vasoactive intestinal peptide, substance P), during the erectile process is also suggested. 6- 8 The role of the central nervous system in penile erection, however, remains relatively unexplored. Administration of vascular smooth muscle relaxants, for example papaverine or prostaglandin El, or a-adrenoceptor antagonists, such as phentolamine, results in penile erection because of an increase in arterial inflow of blood, distension of sinusoids and possible restriction of venous outflow. 9 • 10 Thus, intracavernous injection of vasoactive drugs offers impotent patients another form of therapeutic management, 9- 11 and allows one of the tests for differential diagnosis between vasculogenic and other etiologic forms of impotence. 9 Papaverine and prostaglandin El are also important tools in the assessment of pharmacological response of penile erectile tissues under experimental conditions. 12- 14 Many physiological studies on penile erection were carried out in larger animals, including monkeys, dogs, rabbits and cats. 1 • 3• 4 • 15 • 16 The large volume of tissues and prominent peripheral nervous and blood vessel supplies to the penis in these animals provide convenient access for pharmacological, neurological and hemodynamic evaluations. However, it is becoming financially prohibitive to obtain and maintain monkeys, dogs and cats for experimental purposes. Small laboratory animals such as rats, on the other hand, offer the advantage of Accepted for publication October 23, 1991. * Requests for reprints: Institute of Pharmacology, National YangMing Medical College, Shih-Pai, Taipei 11221, Taiwan, Republic of China.
easy and much less expensive supply of strain-specific and agematched experimental animals for the study of penile erection. Furthermore, the male rat shows many similarities with the behavioral and endocrine aspects of sexual activity in man.17 Therefore, the male rat may be a satisfactory and useful animal model for the investigation of penile erection. We evaluated the feasibility of this model, using directly measured intracavernous pressure in the rat as the experimental index for penile erection. We based our conclusion on the recorded pharmacological responses of rodent penile tissues to papaverine and prostaglandin El, two vasoactive drugs most commonly used in clinical management and diagnosis of impotence. 9- 11 • 18 MATERIALS AND METHODS
Male, adult Sprague-Dawley rats (200 to 300 gm.) anesthetized with pentobarbital sodium (50 mg./kg., i.p., with 10 mg./ kg./hour i.v. supplements) were used. The animal was placed on a heating pad to maintain its body temperature at 37C. The trachea was intubated for better maintenance of airway patency. The femoral artery and vein on one side were cannulated for measurement of arterial pressure and administration of drugs. The former was achieved via a pressure transducer (Gould 23ID) and a Gould pressure processor preamplifier that registers both pulsatile and mean arterial pressures. Heart rate was measured by a tachometer preamplifier (Gould) triggered by the arterial pulses. The skin overlying the penis was incised, and the prepuce was degloved to fully expose both corpora cavernosa. A 26gauge needle filled with saline and connected to a Gould 23ID pressure transducer was carefully inserted into the corpus cavernosum on one side to measure the intracavernous pressure. lntracavernous pressure was monitored alongside pulsatile and mean arterial pressure, and heart rate during the experiment. The effect on the intracavernous pressure of two vasoactive drugs most commonly used in clinical management of impotence, papaverine hydrochloride (U-Liang Pharmaceutical, Taiwan) and prostaglandin El (Ono Pharmaceutical, Japan) was used to evaluate the feasibility of our rat model. Drugs
11 ?,4
1125
RAT MODEL FOR PENILE ERECTION
were given intracavernously via the intracavernous pressure recording needle, at a volume of 0.05 or 0.1 ml. Papaverine (0.05, 0.1, 0.2, 0.4 or 0.8 mg.) or prostaglandin El (1, 2 or 4r µg.) was used to observe the degree and duration of induced changes in intracavernous pressure. Although drugs were given consecutively, a second injection was always delivered only after the intracavernous pressure has returned to baseline level. We also studied the possible detumescent action of clonidine (Jung-Shin Pharmaceutical, Taiwan). 19 The imidazoline compound was injected intracavernously (7.5 or 15 µg.) either subsequent to, or in combination with, papaverine (0.2, 0.4 or 0.8 mg.). Intracavernous injection of saline, at 0.05 and 0.1 ml., was routinely executed at the beginning of the experiment to ascertain that the introduction of fluid directly into the corpus cavernosum may not be a confounding factor. We also monitored the hemodynamic parameters to confirm that the action ofpapaverine, prostaglandin El or clonidine was exerted locally at the cavernous tissue rather than systemically. At the end of each experiment, a small amount of black ink was injected intracavernously to verify the position of the recording needle. The oblique antero-posterior and transverse dimensions of each corpus cavernosum were also measured under a dissecting microscope. All data were presented as mean ± S.E. Differences between treatment groups were evaluated by ANOV A, followed by the Dunnett test for post hoc comparison of means against the control. Values were considered significant at p
