High osmolality: a potent parathyroid hormone secretogogue in dispersed parathyroid cells.

We have examined possible mechanisms by which osmolality might modulate PTH secretion in dispersed bovine parathyroid cells. Increasing medium osmolality by adding sodium chloride causes a marked, dose-dependent increase in PTH release. The maximum effect (4-fold increase) is observed when osmolality is around 650 mosM, with half-maximal stimulation at about 400 mosM. When osmolality is increased to a similar extent with either sucrose or sodium chloride, PTH secretion is enhanced to a comparable degree, suggesting that osmolality itself, rather than ionic strength, is responsible for the increase in PTH secretion. Time course experiments show that the increased secretion of PTH with high osmolality occurs very rapidly (in less than 5 min). In contrast to the suppressive effects of high Ca2+ on PTH release, increasing calcium concentration in the incubation media does not inhibit the stimulation of PTH secretion by high osmolality. Moreover, the effects of dopamine (10(-5) M) and high osmolality on PTH release are additive, further suggesting that high osmolality and cAMP modulate PTH release by different mechanisms. In fact, direct measurement of cellular cAMP in cells exposed to high osmolality shows no change relative to control cells incubated with normal osmolality, 127 +/- 20 vs. 146 +/- 21 fmol/10(5) cells, respectively. Cytosolic Ca2+ increases from 257 +/- 29 nM to 703 +/- 50 nM after 200 mM NaCl is added to the incubation medium at low Ca2+ (0.5mM). Prior removal of extracellular calcium abolished this effect. Increasing the osmolality to a similar level by adding sucrose to the medium does not demonstrate any increase in cytosolic calcium. We conclude that high osmolality is a potent secretogogue in dispersed bovine parathyroid cells. Unlike dopamine and isoproterenol, high osmolality does not act through changes in intracellular cAMP. It also prevents the normal inhibitory effect of high Ca2+ on PTH release. Change of cytosolic Ca2+ is variable and suggests that the effect of high osmolality on PTH release cannot be explained by cytosolic Ca2+ alone. Further understanding of the mechanisms by which osmolality affects PTH release, therefore, may provide clues to the unusual inverse relationship between extracellular and cytosolic calcium and PTH release.