In vitro contractile response of canine urinary bladder tomuscarinic and electrical stimulation: effects of neutering

V.A. Coit, I.F. Gibson, N.P. Evans, F.J. Dowell, Division of Cell Sciences, ICM, Glasgow University Veterinary School, Glasgow, UK, G61 1QH.

Urinary incontinence in the bitch is reported to affect 20% of neutered bitches (Arnold et al., 1989), compared to 0-1% of intact bitches (Holt & Thrusfield, 1993). Currently, the full aetiology of acquired urinary incontinence in the bitch remains undetermined, and the role of the detrusor muscle in the condition is uncertain. The aim of this study was to determine if neutering alters the ability of the canine detrusor muscle to respond to muscarinic and neurogenic electrical stimulation.

Strips of smooth muscle (approx 10mm x 2mm) from the dome of the bladder were dissected from entire and neutered male and female dogs (ME, MN, FE and FN respectively) euthanased by barbiturate overdose, with informed owner consent. The dogs were of varying breeds, weights (9-45kg) and ages (1-14years). The strips were mounted for isometric tension recording in tissue baths under 4g resting tension and bathed in Krebs solution (37ºC, gassed with 95% O2 / 5% CO2). Cumulative concentration response curves were constructed for carbachol (10-9M to 3x10-5M) and frequency response curves to neurogenic stimulation generated (0.01Hz – 100Hz, pulse width 0.5msec, 100 pulses).

Table 1. Maximal tension produced and Log EC50 in response to carbachol and electrical stimulation. Data expressed as g/mg of tissue with results given as mean ± s.e.mean where n = number of animals. * indicates significant difference from entire.

Analyses of variance (Bonferroni post-test) show no effect of weight, age or gender and no interaction between gender and neutering. Neutering is shown to result in a significant reduction in maximal response to both carbachol (p<0.005) and electrical stimulation (p<0.005), and a reduction in sensitivity to carbachol (p<0.001).

These results show that in the dog neutering causes a decreased bladder response and sensitivity to muscarinic stimulation with a concurrent decrease in response to neurogenic electrical field stimulation. It would appear that in neutered dogs there is a change in the muscarinic receptor effector pathway. Whether this may play a role in post neutering urinary incontinence in the bitch remains to be evaluated.

Arnold , S. et al. (1989) Schweiz Arch Tierheilkd . 131(5):259-63.
Holt, P.E., Thrusfield,M.V. (1993) Vet Rec. 133(8):177-80.

Financial support from PetPlan and Pfizer Animal Health is gratefully acknowledged.