Effect of thyroxine on urinary bladder autonomic receptor densities and contractility

The effect of thyroxine on the response of urinary bladder strips to autonomic drugs and electrical stimulation was studied. Bladder strips from euthyroid, hyperthyroid and thyroidectomized rats were used. Rats were made hyperthyroid by intramuscular injection of thyroxine (250 micrograms/rat for 5 days). Surgical thyroidectomy was performed 12-15 weeks before the experiment. The contractile response of bladder strips from each of the 3 groups of rats to electrical field stimulation and the following drugs was measured: isoproterenol; bethanechol; methoxamine; adenosine triphosphate, and potassium chloride. Isoproterenol produced a significantly greater relaxation in strips from hyperthyroid than from euthyroid rats. The enhanced relaxation response to isoproterenol was accompanied by an increase in beta-adrenergic receptor density. The contractile response to methoxamine, adenosine triphosphate and potassium chloride was unchanged by pretreating the rats with thyroxine. The increase in contraction produced by bethanechol was the same in strips from hyperthyroid and euthyroid rats and the muscarinic receptor densities were also unaltered by thyroxine. Bladder strips from thyroidectomized rats showed the same response as strips from control animals following electrical field stimulation or exposure to isoproterenol, bethanechol, methoxamine, adenosine triphosphate and potassium chloride. These results demonstrate that thyroxine pretreatment enhances the relaxation of urinary bladder strips produced by isoproterenol and that this response is correlated with a hormone-induced increase in beta-adrenergic receptors in this tissue.