Stimulus-secretion coupling of parathyroid hormone release: studies of 45Ca and 86Rb fluxes

Pieces of rat parathyroid glands were used to study fluxes of 45Ca and 86Rb. The uptake of 45Ca increased with the extracellular Ca2+ concentration up to at least 5 mM. A rise of extracellular Ca2+ had dual effects on 45Ca efflux in terms of an initial stimulation and a subsequent inhibition. However, K+ depolarization neither affected the uptake nor the efflux of 45Ca indicating a lack of voltage-dependent Ca2+ channels. The depolarization obtained with exposure to Ca2+ cannot be attributed to a decreased K+ permeability, since the 86Rb concentrating ability diminished and the efflux of the isotope increased when parathyroid pieces were exposed to a raised Ca2+ concentration. A stimulation of 86Rb efflux by the Ca2+ ionophore A-23187 indicated that the parathyroid cells possess a K+ permeability activated by cytoplasmic Ca2+. It is suggested that Ca2+ fluxes through channels sensitive to activation by Ca2+ are important both for the membrane potential and the cytoplasmic Ca2+ activity.