Rat anterior pituitaries were incubated in medium containing 45Ca2+, superfused with non-radioactive medium and the efflux of 45Ca2+ studied. 2. When pituitaries from overiectomized rats were used the addition of LH-RH to the medium at a time when slow exponential efflux of Ca2+ was occurring produced a transient increase in 45Ca2+ efflux simultaneous with or before an increased release of LH. The stimulation of both 45Ca2+ efflux and LH release showed a similar concentration dependence on LH-RH. 3. Increasing the K+ concentration of the medium tenfold also stimulated 45Ca2+ efflux and LH release. The response to LH-RH of both parameters was reduced by superfusion with calcium-free medium. 4. LH-RH induced only a small increase in pituitary 45Ca2+ efflux when intact or thyroidectomized male rats were used. TRH increased 45Ca2+ efflux from thyroidectomized rat pituitaries but had only a small effect when pituitaries from intact or ovariectomized rats were studied. 5. Pretreatment of ovariectomized rats with oestradiol inhibited the subsequent increase in 45Ca2+ efflux induced by LH-RH. 6. It is concluded that the secretagogue induced increase in 45Ca2+ efflux results from an increase in cellular Ca2+ activity which is presumably active in stimulus-secretion coupling.
