Parathyroid hormone-related protein (PTHrP) gene expression in the pregnant rat uterus has been shown to be dependent on occupancy of the uterus by the fetus. To further test the hypothesis that the synthesis of PTHrP in smooth muscle tissue is regulated by mechanical stretch, we conducted experiments using the rat urinary bladder as a model of an expansible hollow organ. The results indicate that PTHrP mRNA levels do change in response to the stretch of the bladder wall. Under normal conditions PTHrP mRNA levels in the bladder correlated with the urine volume-namely, the extent of bladder distension. When bladders were maintained empty in vivo, PTHrP mRNA levels decreased gradually. Conversely, when bladders were distended by the accumulation of urine, levels of PTHrP mRNA increased dramatically with time. When distension was limited to one-half of the bladder, the increase in PTHrP mRNA was observed only in the distended portion. Histochemical studies performed on distended bladder tissue indicated the presence of PTHrP immunoreactivity in smooth muscle cells. Isolated organ bath studies were used to examine the possible physiological role of PTHrP in smooth muscle tonicity. In vitro responsiveness of bladder muscle strips to exogenous PTHrP was dependent on the in vivo condition of the bladder. In muscle strips obtained from bladders kept empty in vivo, PTHrP-(1-34)-NH2 relaxed carbachol-induced contraction in a dose-dependent manner but failed to relax the contraction in muscle strips from distended bladders that had high endogenous PTHrP expression. These results and the previous findings in the rat uterus suggest a physiological role of PTHrP in bladder smooth muscle function.