Increased cytosolic calcium stimulates exocytosis in bovine lactotrophs. Direct evidence from changes in membrane capacitance.
The patch-clamp technique has been used to measure changes in membrane capacitance (Cm) of bovine lactotrophs in order to monitor fluctuations in cell surface area associated with exo- and endocytosis. Cells were prepared by an enrichment procedure and cultured for up to 14 d before use. Under whole-cell recording, cell cytoplasm was dialyzed with various Ca2(+)-containing solutions. The resting Cm of 6.05 +/- 1.68 pF was found to correlate well with squared cell radius, suggesting a specific Cm of 0.8 microF/cm2. Discrete Cm steps of 2-10 fF were recorded, which most likely reflect single fusion and retrieval events of prolactin-containing granules (0.2-0.6 microns in diameter). High Ca2+ resulted in a Cm increase of 20-50% from the resting value, demonstrating a role for [Ca2+]i in stimulus-secretion coupling. Spontaneous Cm changes have also been recorded, which presumably reflect prolactin secretion supported by a tonic influx of Ca2+ through the membrane. This is supported by the following findings: addition of Co2+ diminished or reversed the spontaneous Cm changes and decreased resting [Ca2+]i; and membrane depolarization increased Cm, indicating the role of voltage-activated channels in stimulus-secretion coupling. As bovine lactotrophs have been found to be largely devoid of spontaneous electrical activity, a mechanism involving modulation of a tonic Ca2+ influx is proposed; this is shown to provide adequate control of basal and triggered secretion monitored by Cm.
Zorec
R
R
Department of Neuroendocrinology, Institute of Animal Physiology and Genetics Research, Babraham, Cambridge, United Kingdom.
Sikdar
S K
SK
Mason
W T
WT
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002417:Cattle; D003035:Cobalt / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism / Q000502:physiology*; D004594:Electrophysiology; D005089:Exocytosis; D016257:Fura-2; D008564:Membrane Potentials; D010903:Pituitary Gland, Anterior / Q000378:metabolism / Q000502:physiology*
D003035:Cobalt; D002118:Calcium; D016257:Fura-2