Abstract
1 The circulatory actions of dilator substances reflect their relative effectiveness in dilating resistance vessels as compared to the vessels making up the systemic venous reservoir.
2 The ability of dilator substances to act selectively on blood vessels of different types depends on functional differences in the smooth muscle. The smooth muscle of the resistance vessels, for example, depends for its activation on the entry of calcium through potential operated channels and is, in consequence, particularly sensitive to the dilator action of drugs such as verapamil and nifedipine.
3 Drugs which act predominantly upon the resistance vessels cause an increase in heart rate and cardiac output, but this is not, as is commonly supposed, dependent on an increase in sympathetic activity.
4 All directly-acting dilator substances give rise to an increase in plasma renin activity, but their ability to cause sodium retention varies greatly.
5 α-Adrenoceptor antagonists show circulatory effects suggesting a predominant effect upon the venous system after single doses, but their effects are more balanced during chronic administration. They vary in their effect on plasma renin activity, but all tend to cause sodium retention.
6 Tolerance has been demonstrated to the dilator action of the organic nitrates and also of hydralazine.
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Selected References
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