Seasonal variations in the fine structure of the Necturus maculosus urinary bladder epithelium: Low transporters and high transporters

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Abstract
Although the urinary bladder of Necturus maculosus provides an important model system for studying the mechanisms of active Na absorption, little critical attention has been paid to the fine structure of its epithelium. Moreover, two distinct groups of urinary bladders, low and high Na transporters, have been described based on short‐circuit current or transepithelial potential difference. In the present study, over an 11‐month period, stable electrical parameters (short‐circuit current, transepithelial potential difference, and resistance) were recorded from 63 chamber‐mounted bladders. Analysis of these parameters revealed a highly significant difference between two groups (low transporters and high transporters) occurring at different times of the year. Consistent with these data, in urine collected from the bladders, the Na concentration in low transporters was significantly higher than that in high transporters. A subpopulation of these bladders was subsequently fixed and examined at the light and/or electron microscopic level. Low‐transporting bladders were characterized unequivocally by a thin, stratified squamous epithelium only 6–15 μm thick. High‐transporting bladders were composed predominantly of columnar‐shaped granular cells up to 70 μm in height, with ciliated, mitochondria‐rich, and basal cells present in small numbers. There is thus a correlation between transport activity, as measured by electrophysiological techniques and urine sodium analysis, and the structure of the tissue. Moreover, these parameters exhibit significant seasonal variation, the underlying mechanisms of which remain obscure.