Pharmacological characterization of the nucleotide receptors that mobilize Ca2+ ions in human parathyroid cells.

We have used the fluorescent probe fura-2 to perform agonist studies of the receptor(s) that mobilizes Ca2+ ions in response to extracellular ATP in human parathyroid cells. Extracellular ATP induced Ca2+ responses in both normal and adenomatous parathyroid cells. Activation resulted in an initial small transient response during which Ca2+ ions were released from intracellular stores, followed by a prominent plateau response during which Ca2+ ions entered the cells from the extracellular fluid. The responses exhibited moderate desensitization upon repeated stimulation with ATP, and the ratio of the plateau to the peak response remained constant for any given group of activated cells. The baseline intracellular calcium concentration was 100 +/- 4.3 nM (mean +/- S.E.M., n = 3). Following maximal activation by extracellular ATP it rose to a peak of 684 +/- 45.7 nM (n = 3) and a plateau level of 415 +/- 9.9 nM (n = 3). We examined the effects of a variety of nucleotide species. The order of potency was: adenosine, AMP < alpha, beta-methylene ATP < ADP < ATP approximately UTP. In the concentration range 1-1000 microM, UTP (the concentration of agonist inducing a half-maximal response, EC50 = 2.4 microM) was slightly more potent than ATP (EC50 = 3.6 microM), and the two nucleotides evoked similar maximal responses. In the concentration range 0.01-1.0 microM, however, there was a clear difference in the behaviour of the two nucleotides. In particular, ATP, but not UTP, evoked responses that suggested the presence of a second receptor of higher potency but markedly lower efficacy.(ABSTRACT TRUNCATED AT 250 WORDS)