Changes in Precapillary Resistance in Skeletal Muscle Vessels Studied by Intravital Microscopy

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Abstract
Eriksson, E. and B. Lisander.Changes in precapillary resistance in skeletal muscle vessels studied by intravital microscopy.Acta physiol. scand. 1972.84.295– 305.The microcirculation of the cat's tenuissimus muscle was observed with particular regard to internal arterial diameters, linear flow rates and capillary flow distribution. Simultaneous recordings of systemic arterial blood pressure and blood flow in this and adjacent muscles were carried out. The effects of graded vasoconstrictor fibre stimulation or of intra‐arterial vasodilator drug administration could be directly studied in this muscle region. Although causing a slight blood flow increase, acute sympathetic denervation was not followed by any sustained diameter increase in the microvessels studied. Morphologically clearcut precapillary sphincters were not observed and during “rest” the denervated precapillary vessels displayed a fairly steady tone. Nevertheless, capillary flow patterns changed perpetually. Capillary contents were stagnant and flow even reversed sometimes due to transient trapping of passing leucocytes. Maximal drug‐induced dilations caused 50 % diameter increases in arteries of 15–100μ,with smaller changes in larger ones. Intense vasoconstrictor fibre activation lowered flow 6–8 times and reduced diameters in arteries of 15–200μwith a maximum at the 30μsize, where closure could occur. Luminal reductions were absent in immediate precapillary sections. During stimulation a marked leucocytosis was observed in the microvessels. It is concluded that in the muscle studied, lacking morphologicallly distinct precapillary sphincters, variations in capillary blood flow and in the perfused capillary surface area are brought about by luminal changes of end‐arterioles, to which white cell obstruction of microvessels may contribute. Further, the relative importance of the resistance of the different series‐coupled sections may vary considerably with the regional tone. Sections responsible for changes in resistance and in capillary flow distribution are evidently not distinctly separated morphologically.