Intrinsic Tone as Potential Vascular Reserve in Conductance and Resistance Vessels

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Abstract
Background The purpose of this study was to define the degree of intrinsic tone in conductance and resistance vessels, to define the calcium dependency of intrinsic tone in these vascular preparations, and to investigate the efficacy of vasodilatory agents on the level of intrinsic tone in these vascular preparations. Methods and Results All vessels were deendothelialized. Isometric force was recorded from strips of ferret aorta, ferret pulmonary artery, and human coronary artery. Vessel diameter was recorded from the ferret epicardial coronary artery and from ferret coronary microvessel in a pressurized no-flow state. Intrinsic tone was defined as the active increase in force or decrease in diameter with warming from 6°C to 37°C. Changes in force or diameter with various pharmacological agents were expressed as a percentage of intrinsic tone. Our results indicate that intrinsic tone accounts for ≈35% to 40% of total tone in all vascular preparations studied and is not dependent on extracellular calcium. Agents that increased cAMP levels (eg, forskolin, milrinone) and agents that decreased protein kinase C activity (eg, staurosporine) were partially effective in decreasing intrinsic tone. Nitroprusside, adenosine, hydralazine, and nifedipine had no significant effect. Conclusions Our results indicate that intrinsic tone represents a significant component of vascular tone that has not been previously recognized and remains largely unexploited by current pharmacological therapies.