CONTRACTION OF THE SHEEP MIDDLE CEREBRAL, PULMONARY AND CORONARY-ARTERIES INITIATED BY RELEASE OF INTRACELLULAR CALCIUM

1 The aim of the study was to compare contraction initiated by intrace llular Ca2+ release in the middle cerebral, coronary and pulmonary art eries of the sheep. With all three arteries from the sheep, incubation in Ca2+-free physiological salt solution (PSS) reduced agonist-induce d contraction much more than occurred with the rabbit aorta. The intra cellular Ca2+ store appeared to be of limited capacity, since contract ion was transient in Ca2+-free conditions with most agonists. 2 In the middle cerebral artery, contraction in Ca2+-free conditions was much reduced if a previous contraction had been obtained (for 5-hydroxytryp tamine, 5-HT, from 11 +/- 4 to 1 +/- 0.5% of control contraction in 2. 5 mM Ca2+, suggesting that the previous contraction had partly dischar ged the intracellular Ca2+ store. Contraction was less affected in the pulmonary artery and almost unaffected in the coronary artery (for 5- HT, from 15 +/- 1 to 11 +/- 1%) by a previous contraction in Ca2+-free conditions. 3 Rings prepared from small branches of the pulmonary and coronary arteries were affected by Ca2+ deprivation in a similar mann er to large diameter pulmonary and coronary artery rings. 4 In Ca2+-fr ee PSS, contraction induced by prostaglandin E(2) was almost eliminate d (3 +/- 1% of control contraction in 2.5 mM Ca2+, contractions induce d by 5-HT and noradrenaline were reduced, and contraction induced by t he thromboxane mimetic U46619 was least affected (up to 73 +/- 8%). 5 Increasing agonist concentration from EC(50) to the maximally effectiv e concentration raised the percentage contraction remaining in the mid dle cerebral artery (for noradrenaline from 7 +/- 2% to 12 +/- 3%) but not in the pulmonary artery (for noradrenaline from 22 +/- 2% to 24 /- 6%). 6 The present study has revealed notable differences, in coupl ing to intracellular Ca2+ release between the three vascular territori es studied.