RYANODINE RECEPTORS DECANT INTERNAL Ca²⁺ STORE IN HUMAN AND BOVINE AIRWAY SMOOTH MUSCLE

¹ Firestone Institute for Respiratory Health, St. Joseph's Hospital and the Dept of Medicine McMaster University, Hamilton, Ontario, Canada L8N 3Z5
² Toronto Lung Transplant Program, Division of Thoracic Surgery, University of Toronto, Ontario, Canada

^* To whom correspondence should be addressed. E-mail: janssenl{at}mcmaster.ca.

	Abstract

Ryanodine receptors (RyR) in human and bovine airway smooth muscle (ASM) were investigated using a variety of physiological approaches. Using Ca²⁺-indicator dye fluorimetry, we showed RyR to be functional within these tissues, mediating a rapid release of Ca²⁺ from the sarcoplasmic reticulum (SR) in a ryanodine- and chloroethyl phenol- (CEP-) sensitive fashion. Neither ryanodine nor CEP inhibited contractile responses to KCl, cholinergic agonist (acetylcholine or carbachol) or serotonin, indicating no direct role for RyR in contraction; in fact, there was some evidence for augmentation of these responses. In tissues precontracted with carbachol, the concentration-response relationships for isoproterenol or salmeterol were unaffected by ryanodine; relaxations to a nitric oxide donor were also largely unaffected. Finally, we examined whether RyR were involved in regulating [Ca²⁺]_i within the subplasmalemmal space using patch-clamp electrophysiology as well as Ca²⁺ fluorimetry: isoproterenol increased [Ca²⁺]_i and Ca²⁺-dependent K⁺ current activity in a ryanodine-sensitive fashion. We conclude that RyR in human and bovine ASM are not important in directly mediating contraction nor relaxation. We speculate instead that these allow the SR to release Ca²⁺ towards the plasmalemma (to unload an overly full Ca²⁺ store and/or increase the Ca²⁺-buffering capacity of the SR) without affecting bronchomotor tone.