Variability of cellular responsiveness to ADH stimulation in toad urinary bladder

The hydrosmotic response of toad bladder to antidiuretic hormone (ADH) is quantitatively linked to the induced fusion of aggrephores with, and the appearance of aggregates of tightly packed intramembrane particles in the luminal membrane of granular cells. We used these morphological indexes of hormonally induced cell activation 1) to assess the variability of individual cell responsiveness to a maximally stimulating concentration of ADH and 2) to compare cell response patterns in paired tissues where the extent of whole tissue stimulation, as evidenced by transtissue water flow, was either maximal or submaximal. The results indicate that individual cell responsiveness within the same tissue to standardized maximal ADH treatment varies between two- and sevenfold, depending on the morphological endpoint measured. Furthermore, based on skewness in endpoint distribution, this variability appears to reflect inherent heterogeneity of granular cell reactivity to hormone. In relation to proportional tissue responses elicited by different stimulating concentrations of ADH, our observations of luminal membrane aggregate incidence suggest that the responding cells, whatever their sensitivity, participate in a graded, rather than "all-or-none," "on-off" manner.