Effect of norepinephrine on penetrating arterioles of rat cerebral cortex

Penetrating intracerebral arterioles from the rat with mean resting diameters of 26.3 ± 2.8 μm and maximal diameters of 37.4 ± 2.5 (SE) μm were isolated, cannulated, and perfused in vitro. Spontaneous tone development and hydrogen ion reactivity indicated that vessels were relatively undamaged and viable. The norepinephrine (NE) reactivity of intracerebral penetrating arterioles is pH dependent. Vessels studied at extraluminal pH 7.30 were unresponsive to NE up to concentrations of 10^-6 M. At NE concentrations above 10^-6 M, vessels dilated to diameters approximately 110% of control. At pH 7.80, vessels contracted to diameters between 75 and 80% of control in response to extraluminal NE concentrations between 10^-11 and 10^-6 M. Vessels in 10^-6 M phentolamine failed to respond to 10^-8 M NE at pH 7.30 and pH 7.80. An aliquot of 10^-8 M NE solution made up at pH 7.80 failed to induce vessel contraction when the pH was readjusted to 7.30, indicating that a NE breakdown product was not responsible for the contraction. These data suggest that adrenergic mechanisms in penetrating intracerebral arterioles are significantly different from those seen in peripheral arterioles of similar size.