Calcium-dependent transferrin receptor recycling in bovine chromaffin cells

The release of regulated secretory granules is known to be calcium dependent. To examine the Ca2+-dependence of other exocytic fusion events, transferrin recycling in bovine chromaffin cells was examined. Internalised I-125-transferrin was released constitutively from cells with a half-time of about 7 min. Secretagogues that triggered catecholamine secretion doubled the rate of I-125-transferrin release, the time courses of the two triggered secretory responses being similar. The triggered I-125-transferrin release came from recycling endosomes rather than from sorting endosomes or a triggered secretory vesicle pool. Triggered I-125-transferrin release, like catecholamine secretion from the same cells, was calcium dependent but the affinities for calcium were very different. The extracellular calcium concentrations that gave rise to half-maximal evoked secretion were 0.1 mm for I-125-transferrin and 1.0 mm for catecholamine, and the intracellular concentrations were 0.1 mum and 1 mum, respectively. There was significant I-125-transferrin recycling in the virtual absence of intracellular C-a2+, but the rate increased when Ca2+ was raised above 1 nm, and peaked at 1 mum when the rate had doubled. Botulinum toxin type D blocked both transferrin recycling and catecholamine secretion. These results indicate that a major component of the vesicular transport required for the constitutive recycling of transferrin in quiescent cells is calcium dependent and thus under physiological control, and also that some of the molecular machinery involved in transferrin recycling/fusion processes is shared with that for triggered neurosecretion.