Autoregulation during pressor response elevates wall shear rate in arterioles

Autoregulation of blood flow implies reciprocal changes in vessel diameter and red blood cell velocity (VRBC) when perfusion pressure is altered. We tested two hypotheses: 1) blood flow will be autoregulated throughout arteriolar networks during a pressor response, and 2) wall shear rate (WSR; proportional to VRBC/diameter) will increase during autoregulation. Male hamsters (109 +/- 3 g; n = 22) were anesthetized (pentobarbital sodium 60 mg/kg), and the cremaster muscle was prepared for intravital videomicroscopy. Internal diameter and VRBC were monitored in first (1A)- through fourth (4A)-order arterioles; WSR and blood flow were calculated. Data were acquired at rest and at the peak of diameter responses to bilateral carotid artery occlusion (CAO). At rest, 1) mean arterial and 1A transmural pressures were 100 +/- 5 and 59 +/- 4 mmHg, respectively; 2) as branch order increased, arteriolar diameter, VRBC, and blood flow decreased (P < 0.05); and 3) WSR and resting tone increased with branch order (P < 0.05). During pressor responses to CAO, 1) arterial and 1A pressures increased to 145 +/- 7 and 89 +/- 5 mmHg, respectively (P < 0.05); 2) 1A branches dilated while 2A, 3A, and 4A branches constricted (P < 0.05); 3) VRBC and WSR increased in all branches (P<0.05); and 4) blood flow increased in 1A and 2A branches (P < 0.05), yet was unchanged (i.e., was autoregulated) in 3A and 4A branches. Arteriolar constrictions during CAO were not affected by alpha-adrenoceptor blockade with phentolamine (10(-6) M). We conclude that autoregulation of muscle blood flow during a pressor response involves myogenic constriction of arterioles with concomitant elevation of WSR.