Ion channels in urinary bladder.

Urinary epithelia separate urine from interstitial fluid. In the mammal, this tight epithelium has a limited transport capacity but is capable of moving sodium from urine to blood through an aldosterone-sensitive cellular pathway. In lower vertebrates, absorption of ions and water from the urine can contribute significantly to fluid and electrolyte homeostasis. Transepithelial ion transport and maintenance of cellular composition are interdependent, requiring a balance between movements across the apical and basolateral plasma membranes through a variety of pathways including electrodiffusion through ion channels. A variety of such channels has been identified in urinary epithelia. Apical membranes contain amiloride-sensitive, highly selective sodium channels of low conductance (approximately 5-10 pS). There is evidence that in mammalian bladders trypsin-like enzymes in the urine continually degrade these channels, decrease in cation selectivity being followed by loss of the channels from the membrane. New channels stored in the cytoplasm appear to provide a source for replenishment of the membrane. Other channels of higher conductance and lower selectivity have also been described in both mammalian and amphibian bladders, but their physiological significance remains to be established. Basolateral membranes contain potassium channels. In the mammalian bladder, in which chloride appears to be distributed at electrochemical equilibrium, chloride conductance exceeds potassium conductance and patch clamp studies have revealed a chloride channel of conductance approximately 60 pS detectable immediately on patch excision and active at normal membrane potentials. In the amphibian bladder, a variety of findings indicates the presence of a basolateral membrane chloride conductance, but patch clamp data are not yet available.