TOXINS AS TOOLS TO CHARACTERIZE THE ION CHANNELS UNDERLYING STIMULUS-SECRETION COUPLING IN PITUITARY MELANOTROPHS

POSTER PRESENTATIONS

Objective. Identify the ion channels involved in stimulus-secretion coupling in melanotrophs by the use of toxins. Background. Hormone secretion from melanotrophs is dependent on Ca2+ entry through voltage-gated channels. Since melanotrophs are electrically excitable and exhibit spontaneous action potentials, the effects, on secretion, of toxins which alter the activity of voltage-gated channels were investigated. Methods. Melanotrophs were placed in a perifusion chamber and melanophore-stimulating hormone content of the perifusate was measured using the Anolis skin bioassay. Toxins did not affect the assay. Results. Tetrodotoxin, which blocks voltage-gated Na channels and abolishes Na spikes in melanotrophs, had no effect on basal secretion. FTX-3.3 blocks low threshold, T-type, Ca channels and inhibited basal but not K-stimulated secretion. FS-2, a blocker of high threshold, L-type, Ca channels blocked K-stimulated secretion but not basal secretion. Two other high threshold Ca channels blockers, omega-Agatoxin (P/Q-type channels) and omega-Conotoxin GVIA (N-type channel) did not affect either basal or K-stimulated secretion. Conclusion. The lack of effect of TTX on basal secretion 75 indicates that the Ca dependence of this secretion is not due to Ca2+ entry through voltage-gated channels opened by Na spiking. However, block of T-type Ca channels, which are active around resting potential, inhibited basal secretion implying that Ca2+ entry through T-type channels supports basal secretion. The inhibition of K-stimulated MSH secretion by an L-type Ca channel blocker and the lack of effect of P/Q and N-type blockers suggest that K-stimulated secretion is mediated by Ca2+ entry solely through high threshold, L-type Ca channels. Grants. Health Professions Division Research Award