Effect of tetrodotoxin on the phasic and tonic responses of isolated rabbit urinary bladder smooth muscle to field stimulation.

The response of the rabbit urinary bladder to field stimulation (80 volts, 2-32 Hz, 1 msec duration) is biphasic, consisting of an initial phasic contraction mediated by cholinergic and purinergic neurotransmitters, followed by a prolonged tonic contraction which is solely cholinergic. Obstructive hypertrophy of the bladder induces a variety of contractile alterations including a significantly greater reduction in the tonic component of the contractile response as compared to the phasic component. This results in a severe dysfunction in the ability of the bladder to empty. One possibility is that the inability of the bladder to maintain tension and empty efficiently may be related to a degeneration of nerves innervating the bladder smooth muscle. In addition to the well documented neuropathy, the bladder undergoes hypertrophy +/- hyperplasia of both smooth muscle and interstitial cellular elements, alterations in the metabolism of substrates, alterations in the synthesis of structural and smooth muscle proteins, and alterations in the deposition of collagen. The purpose of this study was to 1) to create a specific neuropathy in the absence of the additional structural, smooth muscle, and metabolic changes that are induced by partial outlet obstruction; and 2) determine if the contractile dysfunctions induced by the neuropathy have properties similar to the contractile dysfunctions induced by outlet obstruction. In the present study, a progressive "smooth muscle neuropathy" was induced in isolated strips of male rabbit urinary bladder smooth muscle by incubating isolated strips of urinary bladder body in the presence of increasing concentrations of tetrodotoxin (15-1500 nM). In these studies, we determined the effect of increasing concentrations of tetrodotoxin (TTX) on the response to field stimulation utilizing 2 Hz and 32 Hz, at 80 V and 1 ms duration. The effects of TTX on maximum rate of contraction, peak contraction and tonic contraction were monitored. In addition, the effects of atropine (cholinergic muscarinic blockage) and ATP-desensitization (purinergic inhibition) on the effects of TTX were also determined. The results can be summarized as follows: 1) Both atropine and ATP desensitization individually inhibited significantly the peak response to field stimulation. 2) Atropine abolished the tonic response. 3) TTX inhibited the tonic contraction at significantly lower concentrations than it inhibited peak contraction. Thus, at low concentrations of TTX, a condition similar to that seen in obstructive hypertrophy was created. 4) The ED50 in the presence of atropine was significantly greater than the ED50 following ATP desensitization. This may indicate that there are separate synaptic elements for cholinergic and purinergic transmission.(ABSTRACT TRUNCATED AT 400 WORDS)