REGULATION OF L-TYPE VOLTAGE-GATED CALCIUM CHANNELS BY EPIDERMAL GROWTH-FACTOR

GH3 rat pituitary cells have L-type voltage-gated calcium channels, and both the rate of uptake of Ca-45(2+) and the concentration of intracellular free calcium ion ([Ca2+]i) are increased by depolarization with high potassium. Cells incubated for several days with 10 nM epidermal growth factor (EGF) responded to depolarization with a 30-65% smaller increase in Ca-45(2+) uptake than untreated cells. The inhibitory response to EGF developed slowly, with a maximal effect requiring 24-48 h. EGF exerted a half-maximal reduction in depolarization-stimulated Ca-45(2+) uptake at 0.1 nM and a maximal effect at 1-10 nM. Ca-45(2+) uptake was reduced by EGF at strongly depolarized potentials (added K+, > 25 mM) with or without the calcium channel agonist BAY K8644. [Ca2+]i was measured using fura-2 before and after depolarization in control cells and cells incubated for 48 h with 10 nM EGF. EGF-treated cells responded to the addition of 30-50 mM KCl with a smaller increase in [Ca2+]i than control cells. Digital fluorescence imaging of individual fura-2-loaded cells confirmed that the average [Ca2+]i response to depolarization was lower in cells that had been incubated with EGF for 36 h. EGF treatment increased the amount of PRL secreted basally, but inhibited the acute PRL secretory response to depolarization with 50 mM KCl and 1 muM BAY K8644 from 2.6- to 1.5-fold. The results indicate that EGF reduces the activity of voltage-gated dihydropyridine-sensitive calcium channels on pituitary cells, and that this reduction in L-channel activity is accompanied by a smaller secretory response to depolarization.