Comparison of the myogenic response in rat cerebral arteries of different calibers.

The myogenic response, the characteristic of blood vessels to contract with increasing pressure, was studied at three different locations along the middle cerebral artery (MCA) vascular tree. We hypothesized that smaller caliber vessels would have a more pronounced myogenic response at lower pressures than larger diameter arteries, corresponding to pressures normally experienced in vivo. Cerebral vessels (MCAs, branches of the MCA, and penetrating arterioles) were isolated from male rats, cannulated with glass micropipettes, and pressurized. Changes in diameter were measured as the transmural pressure was increased from 20-100 mmHg. The MCAs, which had a resting diameter of 202 +/- 10 micron (n = 9) at 50 mmHg, showed its greatest myogenic response between 60-100 mmHg (8+/-2% constriction, n = 9, p < 0.001). The penetrating arterioles [58 +/- 4 micron (n = 8) at 50 mmHg], on the other hand, showed its greatest myogenic response between 20-60 mmHg (10 +/- 4% constriction, n = 8, p < 0.05). Branches of the MCA [118 +/- 14 micron (n = 8) at 50 mmHg] showed a slight constriction over the entire pressure range (5 +/- 9% constriction between 20-100 mmHg, p=ns). Our results suggest that the myogenic response appears to be best developed in the range of pressures found during physiological conditions for a given vessel in the MCA territory. This characteristic is fundamental in the overall control of cerebrovascular resistance.