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Evoked and Spontaneous Purinergic Junctional Ca²⁺ Transients (jCaTs) in Rat Small Arteries

From the Department of Physiology, School of Medicine, University of Maryland, Baltimore, Md.

Correspondence to Dr W. Gil Wier, Dept of Physiology, University of Maryland, Baltimore, 655 W Baltimore St, Baltimore, MD 21201. E-mail gwier001{at}umaryland.edu

Abstract

Confocal microscopy of fluo-4 fluorescence in pressurized rat mesenteric small arteries subjected to low-frequency electrical field stimulation revealed Ca²⁺ transients in perivascular nerves and novel, spatially localized Ca²⁺ transients in adjacent smooth muscle cells. These muscle Ca²⁺ transients occur with a very brief latency to the stimulus pulse (most <3 ms). They are wider (5 µm) and last longer (t_1/2, 145 ms) than Ca²⁺ sparks. They are abolished by the purinergic receptor (P2X) antagonist suramin, but they are totally unaffected by the ₁-adrenoceptor antagonist prazosin or by capsaicin (which inhibits the function of perivascular sensory nerves). We conclude that these novel Ca²⁺ transients represent Ca²⁺ entering smooth muscle cells through P2X receptors activated by ATP released from sympathetic nerves, and we therefore call them "junctional Ca²⁺ transients" or jCaTs. As expected from spontaneous neurotransmitter release, jCaTs also occur spontaneously, with characteristics identical to evoked jCaTs. Visualization of sympathetic neurotransmission shows that purinergic components dominate at low frequencies of sympathetic nerve fiber activation.

Key Words: calcium transient • smooth muscle • artery

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