The urothelium of a hibernator: the American black bear
By means of a transaortal injection of a gelatine ink mixture, a manifold mucosa to muscularis blood flow ratio was proved. In addition, the treatment of hypo- and hypertonic NaCl by the rat urinary bladder has been studied at 0.3-, 0.6- and 0.9-ml filling levels in conjunction with continuous bladder pressure recording. With distension an increased permeability to NaCl (efflux) and/or water (influx) was found in hypertonic conditions. In order to demonstrate this, the decreasing surface to volume ratio with distension has to be considered. Final urea concentrations in hypertonic media significantly exceeded those in hypotonic probes. The phenomenon has been hypothetically attributed to the existence of an arteriovenous counter current exchange within mucosal vessels. In comparison to hypotonic bladder contents, hypertonic media increased basic bladder pressures and phasic pressure amplitudes preferably at the 0.9-ml level. Hence, in context with an increased permeability, distension favors access of the bladder content to detrusor nerve and muscle cells thereby facilitating their excitability.