Release of a Common Source of Intracellular Ca2+ by α-Adrenergic Agonists and Dinitrophenol in Rat Liver Slices

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Abstract
The effects of α-adrenergic agonists on 45Ca efflux from slices of rat liver were studied and the results compared to earlier studies on the rabbit aorta. Norepinephrine and phenylephrine (PE) cause a stimulation of 45Ca efflux which was due to α1-receptor activation. The mitochondrial uncoupler dinitrophenol (DNP) also stimulated 45Ca efflux while caffeine had little or no effect. PE and DNP effects were due to the release of intracellular 45Ca stores, and both agents released a similar quantity of Ca2+. 45Ca/40Ca exchange in both PE- and DNP-released sources was similar (t½ ≅ 16 min). DNP and PE effects were not additive, and previous exposure to one agent reduces the response to the other. These results suggest that α1-agonists and DNP release a common source of intracellular Ca2+ in rat liver. Since in rabbit aorta α-agonists primarily release Ca2+ from a nonmitochondrial source, our results suggest α1-receptors act via the generation of a Ca2+-releasing substance which subsequently mobilizes Ca2+ from different organelles in different tissues.