PARATHYROID-HORMONE IS ABLE TO ENHANCE CYCLIC ADENOSINE-MONOPHOSPHATEFORMATION WITHOUT CAUSING AN INCREASE IN CYTOPLASMIC CA2+ IN OSTEOBLASTS

There are several reports indicating that parathyroid hormone (PTH), b esides inducing the formation of cyclic adenosine monophosphate (cAMP) , also causes an increase in cytoplasmic free Ca2+ ([Ca2+]i) in osteob lasts, and it has been speculated that both of these second messengers are necessary to mediate PTH-induced bone resorption. In the osteobla stic cell line MC3T3-El, bovine PTH 1-34 (10 nmol/l-1 mumol/l) stimula ted cAMP formation but did not cause an increase in [Ca2+]i in adheren t single cells (basal [Ca2+]i = 151 +/- 5 nmol/l, mean +/- SEM; N = 98 ). In contrast, subsequent addition of bradykinin (1 mumol/l) resulted in a transient increase in [Ca2+ from a basal level of 155 +/- 11 nmo l/l to a peak value of 351 +/- 60 nmol/l (N = 14). When the PTH challe nge was followed by the addition of thrombin (10 U/ml), the latter ind uced a transient rise in [Ca2+]i from a basal level of 173 +/- 12 nmol /l to a peak at 341 +/- 33 nmol/l (N = 20). Primary cultures of human osteoblasts were obtained from trabecular bone. These cells were also PTH-responsive in terms of cAMP formation. On the other hand, human PT H 1-34 (100 nmol/l) did not affect [Ca2+]i in the isolated human osteo blasts, while bradykinin (1 mumol/l) caused a transient increase in [C a2+]i (from a basal value of [Ca2+]i at 154 +/- 10 nmol/l to a peak va lue of 757 +/- 147 nmol/l within 30s; N = 16). Neither in the human os teosarcoma cell line SaOS-2 (basal value of [Ca2+]i at 94 +/- 10 nmol/ l; N = 24), nor in the rat osteosarcoma cell line ROS 1712.8 (basal va lue of [Ca2+]i at 85 +/- 9 nmol/l; N = 9) was any effect of PTH (0.1 n mol/l-1 mumol/l) on [Ca2+]i demonstrated. In conclusion, we were unabl e to detect any effect of PTH on [Ca2+]i in single MC3T3-E1 cells, in isolated human osteoblasts, in SaOS-2 or in ROS 17/2.8 cells, whereas accumulation of cAMP was always seen. This indicates that cAMP is the major second messenger for PTH in osteoblasts.