Augmentation of bladder contractility after beta-adrenergic blockade in spinal cord injured patients

Young rats exhibit an age- and sex-dependent bladder response to alpha-antagonists but not beta-agonists

INTRODUCTION

Previous studies have suggested that the onset of alpha- (α) and beta- (β) adrenergic receptor activity is delayed in young animals. The use of alpha1- (α1-) antagonists for dysfunctional voiding, and beta3- (β3-) agonists for overactive bladder in younger children may not be indicated if maturation is required before bladder and urethral adrenergic receptors are active.

OBJECTIVE

To determine the sex- and age-dependent responses of the bladder and external urethral sphincter (EUS) to α- and β-adrenergic agents in neonatal and young adult rats.

MATERIALS AND METHODS

A total of 72 naïve Sprague-Dawley rats (36 female, 36 male) and 15 bladder-reduced (BR) female Sprague-Dawley rats underwent cystometry and EUS electromyography at 3, 6, and 9 weeks of life. Following administration of WAY 100,635 (0.3 mg/kg, serotonergic receptor antagonist), the non-selective α-agonist phenylephrine (0.3 mg/kg), α-antagonist phentolamine (1-3 mg/kg), β-agonist isoprenaline (3 mg/kg) and β-antagonist propranolol (3 mg/kg) were delivered intravenously. The maximum intravesical pressure (IVP), pressure threshold (PT), intermicturition interval (IMI), contraction duration (CD), burst amplitude and burst frequency were compared after each drug.

RESULTS

The α-antagonist phentolamine lowered the IVP in 9-week-old males without lowering the PT. In contrast, the β-agonist isoprenaline lowered the IVP in both males and females of all ages, also without affecting the PT. Isoprenaline was also effective at shortening the CD in females, suggesting more effective bladder emptying. The α-agonist phenylephrine increased the IVP in 3-week-old and 6-week-old females and 3-week-old males, but this effect was blocked by pretreatment with phentolamine. The β-antagonist propranolol increased the PT in both males and females, and shortened the IMI in females, which was consistent with retention. Phenylephrine increased the burst duration in 9-week-old naïve females, while isoprenaline increased the burst amplitude and duration in 9-week-old BR females.

CONCLUSIONS

In the neonatal rat, both α- and β-adrenergic receptors actively regulate bladder function by 3 weeks of life, but the desired effect of decreasing IVP by α-antagonists was delayed until 9 weeks in male rats. In contrast, β-agonists were effective at decreasing IVP in both male and female rats of all ages, which suggests that they are better agents for enhancing bladder emptying in female and young male rats.

Regulation of bladder and colon--rectum in patients with spinal lesions

Parasympathetic cholinergic stimulation is excitatory for bladder and colon--rectum, normal as well as uninhibited. This is documented by pharmacological and neuroanatomical studies in man. The bladder and the left half of colon and rectum have motor supply in common as demonstrated by surgical intervention on sacral nerves. More detailed studies can be performed by electrical stimulation of sacral roots or rootlets. Sympathectomy may cause alteration of voiding pattern, but often only temporarily, and generally with no alteration of defecation habits. The sympathetic innervation is sparse in the human bladder, but alpha- and beta-adrenergic receptors have been demonstrated. The alpha-adrenergic blocking agent thymoxamine is able to cause a shift in the cystometrogram to the right, and to relax the spastic pelvic sphincters by i.v. administration. Like other alpha-adrenergic blocking agents, it can relax the proximal part of the urethra. The effect of other stimulating and blocking adrenergic compounds are reported, but further studies are needed. No simultaneous reactions are generally recorded in the uninhibited bladder and rectum, and no correlation was found between the hyperactivity of the uninhibited neurogenic bladder and colon.