Urethane is the most suitable anesthetic for acute and chronic experiments of the lower urinary tract that require demonstration of the micturition reflex in rodents because, although there are some changes in bladder function, the micturition reflex is not suppressed [1]. Meanwhile, isoflurane exhibits significant suppression of external urethral sphincter–electromyogram activity and prolonged suppression of the micturition reflex compared with urethane [2]. However, there have not been studies that directly compare the effects of isoflurane and urethane on bladder function. In the present study, we compared the effects of two anesthetics, isoflurane and urethane on bladder function in rats. Arterial pressure, cystometry (CMG), and rhythmic bladder contractions (RBCs) under isovolumetric conditions, mechanosensitive single-unit afferent activities (SAAs), bladder compliance and bladder myogenic microcontractions (bladder microcontractions), bladder blood flow, and blood and urine biochemical tests were investigated in isoflurane- or urethane-anesthetized female rats.

Female Sprague-Dawley rats were used (9-11 weeks old). Rats were divided into two groups and each group received different anesthetic conditions: isoflurane or urethane anesthesia (isoflurane: 5% at initiation, 1.5%–2% at surgery, and 1.1%–1.3% at measurement (room air); urethane: 1.2 g/kg, intraperitoneally). Measurements were taken at least 3 h after the initiation of anesthesia.
Continuous CMG measurements were performed with intravesical saline instillation (6 mL/h). During CMG measurements, systemic arterial pressure was also recorded.
In the RBCs under isovolumetric conditions, after the bladder had been emptied, saline was instilled into the bladder at a rate of 2 mL/h until RBCs were induced. After RBCs were reproducibly induced for a period of 10 min, atropine (1 mg/kg) followed by tetrodotoxin (TTX: 15 μg/kg) were administered intra-arterially (i.a.).
In the SAAs measurements, the fine filaments were dissected from the left L6 dorsal roots and placed across a bipolar electrode for monitoring SAAs. Nerve fibers primarily originating from the bladder were identified by electrical stimulation of the left pelvic nerve and by bladder distension. Nerves with conduction velocities (CV) more than 2.5 m/s were designated as Aδ-fibers and those with CV less than 2.5 m/s as C-fibers. Saline instilled until the intravesical pressure reached 30 cmH2O. The numeric values of Aδ- and C-fiber activities, bladder compliance (mL/Δcm H2O), and the number of bladder microcontractions (>1.5 cm H2O of amplitude) were calculated at the start and the end of the instilling phase.
Bladder blood flow was measured by a laser blood flowmeter. The surface of the bladder was diffusely illuminated with a 780-nm semiconductor laser, scattered light was filtered with a hybrid filter and detected by a charge-coupled-device camera, and the images were transferred to a computer for analysis.
Blood (serum) and urine biochemical tests were additionally performed.

In results of the CMG, three types were found in the isoflurane group, while only one type was found in the urethane group: 3/8 rats in the isoflurane group and 7/7 rats in the urethane group showed constant bladder neurogenic contractions for micturition (figures 1AD), whereas 5/8 rats in the isoflurane group showed unstable contractions (figure 1B) or overflow incontinence (figure 1C). The RBCs appeared in the urethane group but not in the isoflurane group, and RBCs were attenuated by atropine and eliminated by TTX, but fluctuations (microcontractions) were not affected by these drugs (figures 1EF). SAAs in both the Aδ- and C-fibers, bladder compliance, and bladder microcontractions in the isoflurane group were higher than those in the urethane group during bladder distension (figures 2A-H). The blood biochemical test showed that the serum calcium level was higher in the isoflurane group. The mean arterial pressure and bladder blood flow were not different between the groups (figures 1A-D and 2I).

The present study showed that urethane anesthesia retains bladder neurogenic contractions for micturition, whereas isoflurane anesthesia attenuated these contractions. However, bladder compliance, bladder myogenic microcontractions, and the mechanosensitive afferent activities of Aδ- and C-fibers during the storage phase were retained under isoflurane anesthesia, but attenuated under urethane anesthesia.

The results suggest that urethane anesthesia is useful for pharmacological and physiological investigations regarding the bladder contractions for micturition, while isoflurane anesthesia is useful for those regarding the bladder function during the storage phase.

Matsuura S, Downie JW. Effect of anesthetics on reflex micturition in the chronic cannula-implanted rat. Neurourol Urodyn. 2000;19(1): 87-99.
Chang HY, Havton LA. Differential effects of urethane and isoflurane on external urethral sphincter electromyography and cystometry in rats. Am J Physiol Renal Physiol. 2008;295(4): F1248-1253.