Structure and function of small arteries

The small arteries (prearteriolar vessels with lumen diameter less than approximately 500 microns) contribute importantly to and participate actively in the regulation of the peripheral resistance. New techniques, building on the classic histological and hemodynamic techniques, have enabled detailed in vitro investigation of small arteries. At present, research in small arteries is in its infancy, and our understanding of the heterogeneity of small arteries within vascular beds, between vascular beds, and between species is extremely limited. This review attempts to describe the current status of the field. New techniques, based primarily on a wire myograph (where the vessels are mounted as ring preparations) and a pressure myograph (where vessels are cannulated and pressure-lumen relations are determined), have allowed in vitro investigations of small arteries. The more physiological arrangement of the pressure myograph allows, for example, investigation of the vasoconstrictor response to raised intravascular pressure (the Bayliss response), whereas the less-sophisticated wire myograph is similar to use and may be more useful in certain situations where particular mechanisms are being investigated. Both techniques allow simultaneous measurements of vessel tone and a variety of parameters (e.g., membrane potential and intracellular ion activities) and thus allow precise determination of the relation between small artery structure and function. The vessels appear to remain fully viable with regard to the contractility of their smooth muscle cells as well as to the function of their perivascular nerves and their endothelium. The evidence suggests that the monovalent transport mechanisms in the plasma membrane, in particular potassium channels, play an important role in the determination of the membrane potential in small arteries, although the relation is more complex than indicated by the Goldman equation. Confirmation of these findings requires, however, simultaneous determinations of ion transport and vascular tone under conditions where vessels are subjected to mechanical loading. The membrane potential, through its effect on potential-dependent calcium channels, plays an important role in the determination of vascular tone. With regard to calcium homeostasis, current knowledge is hampered by the lack of direct measurements of the relation between cytoplasmic calcium and vascular tone. The evidence, however, suggests that besides potential-dependent calcium channels, receptor-operated calcium channels are present in the plasma membrane, although this still requires confirmation. The role of the sarcoplasmic reticulum is not clarified.(ABSTRACT TRUNCATED AT 400 WORDS)