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Division of Molecular Cardiology, Kyushu University, Fukuoka, Japan.

1. We determined the effects of a change in preload, or resting load, on the development of force and on cytosolic calcium concentration ([Ca2+]i) during the contraction induced by high K+ depolarization and by the stable analogue of thromboxane A2, U-46619, using front-surface fluorometry and medial strips of bovine middle cerebral artery loaded with fura-2. 2. Increase in resting load of strips from 0.495 mN (low load; 0.85L(max)) to 1.98 mN (optimal load; L(max)) elevated resting levels of [Ca2+]i slightly, and, significantly, after a delay of a few seconds. The force developed by high K+ depolarization at resting load of 1.98 mN was much greater than that at a resting load of 0.495 mN (191.8% of that at 0.495 mN); however, there was no difference in [Ca2+]i elevation induced by high K+ depolarization between resting loads of 0.495 and 1.98 mN. 3. Both in the presence and absence of extracellular Ca2+, the force developed by U-46619 (0.1 microM) at resting load of 1.98 mN was also much greater than that at a resting load of 0.495 mN. Both in the presence and absence of extracellular Ca2+, there was no difference in the [Ca2+]i transients induced by U-46619 between two different resting loads. 4. The [Ca2+]i-force relationship during the contraction induced by high K+ depolarization was shifted to the left when the resting load was increased from 0.495 to 1.98 mN. At 0.495 mN resting load, this Ca(2+)-force relationship was shifted to the left by U-46619. This left-side shift of the curve by U-46619 was further enhanced by the increase in resting load from 0.495 to 1.98 mN. 5. The augmentation of force development induced by the change in resting load (from 0.495 to 1.98 mN) was not affected by a relatively specific inhibitor of protein kinase C, 1-(5-isoquinolinesulphonyl)-2-methylpiperazine dihydro-chloride (H-7; 10 microM). 6. We conclude that (1) at a given degree of [Ca2+]i elevation, the developed force induced by U-46619 was greater than that induced by high K+ depolarization, and (2) preload, or resting load (below optimal load), regulates contractile responsiveness of cerebrovascular smooth muscle to various stimulations, mainly by modulating the [Ca2+]i-force relationship, without affecting the extent of [Ca2+]i elevation. The results suggest the possibility that Ca(2+)-insensitive pathways may be involved in the stretch-dependent regulation of Ca2+ sensitivity of the contractile apparatus in smooth muscle.

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