Influence of oligopeptide aldehydes on intracellular Ca2+ concentration in rat pituitary cells

We investigated the effects of some synthetic tripeptide aldehydes, earlier shown to influence pituitary hormone secretion and ⁴⁵Ca²⁺ uptake, on the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) of rat anterior pituitary cells in suspension. Boc-D-Phen- Leu-Phenylalaninal or Boc-D-Phe-Leu-Prolinal in the tested range of 1–100 or 200 μM, respectively, were ineffective in influencing basal [Ca²⁺]_i but caused a concentration-dependent inhibition in K⁺ (25 mM)-induced [Ca²⁺]_i elevation. The IC₅₀ of both effects was about 50 μM. In contrast, they did not interfere with the stimulation caused by the calcium channel agonist BAY K 8644 and were also ineffective in influencing the receptor-mediated stimulus of thyrotropin-releasing hormone on [Ca²⁺]_i. On the basis of the present and foregoing results the possible involvement of calcium channels is discussed, but different mechanisms mediating the tripeptide aldehyde inhibition are also considered. A third tripeptide aldehyde. Boc-Gln-Leu-Lysinal (Boc-GLL), showed ionophore-like properties. This nontoxic substance caused a dose-dependent rise up to 400% (at 100 μM) in [Ca²⁺]_i. Its effect is not mediated by voltage-dependent calcium channels, as it cannot be inhibited either by the classicalp calcium channel antagonists verapamil and nifedipine, or by the above-mentioned inhibitory tripeptide aldehydes. When we decreased the extracellular Ca²⁺ concentration by the addition of 4 mM EGTA, the effect was inverted and Boc-GLL caused a large fall in [Ca²⁺]_i. We suggest that Boc-GLL may open cell membrane pores through which Ca²⁺ moves along the concentration gradient. The calcium flux can be inhibited by 20 mM Mg²⁺ and 100 μM Co²⁺ but not by 500 μM La³⁺. Thus, tripeptide aldehydes. depending on their structure, may decrease or increase [Ca²⁺]_i via uncoventional mechanisms and may serve as tools for dissecting details of cell calcium homeostasis.