Hormonal regulation of gluconeogenesis in cultured proximal tubular cells: role of cytosolic calcium.

Gluconeogenic competence of primary cultures of canine renal proximal tubular cells has been examined. Cells grown in 5 mM glucose media or in glucose-free media exhibited pyruvate-stimulated glucose production, as opposed to cells grown in 20 mM glucose media. By 72 h after the last media change, confluent cells grown in 5 mM glucose medium turn from a predominantly glycolytic to an oxidative type of metabolism. By this time, glucose production exhibited pH, 3-mercaptopicolinate, and insulin sensitivity. Parathyroid hormone, angiotensin II, and phenylephrine stimulated glucose production in a nonadditive fashion. Single-cell cystolic Ca2+ measurements using microspectrofluorometric techniques revealed that all three hormones elicited Ca2+ transients in proximal tubular cells. Ionomycin stimulated glucose production by proximal tubular cells, suggesting that Ca2+ transients could represent a sufficient stimulus for glucose production. When hormone-induced Ca2+ transients were curtailed by a pretreatment with the membrane-permeant Ca2+ chelator, Maptam, hormonal stimulation of glucose production was abolished, suggesting that Ca2+ transients represent not only a sufficient, but a necessary event in the stimulation of glucose production by these hormones.