The effects of and the interaction between local perivascular variation in potassium and bicarbonate concentration on the diameter of pial arteries were studied in cats by the microapplication technique. At 11 mEq/liter of bicarbonate, a direct correlation existed between vascular diameter and K +

The influence of norepinephrine on the diameter of single pial arteries and arterioles was investigated by adding the drug to the perivascular space with micropipettes. The mock spinal fluid solution in which the norepinephrine was dissolved contained 0, 11, or 22 mEq/liter of bicarbonate. These concentrations of bicarbonate were by themselves found to dilate, cause no change, and constrict the pial vessels, respectively. Concentration-response curves with 11 mEq/liter of bicarbonate over the concentration range of 5 X 1O"* to 5 mmoles/liter of norepinephrine showed significant constriction at 5 X 10~2 mmoles/liter and maximal constriction (40% of diameter) at 2.5 mmoles/liter. In bicarbonate-free solution, the slope of the concentration-response curves was less, and at 22 mEq/liter of bicarbonate norepinephrine had no effect. The pial arteries seemed to be less sensitive than the mesenteric and the cremasteric arteries to norepinephrine. The present data demonstrate the existence of norepinephrine receptors on the pial arterial smooth muscle cells, which satisfies the major requirement for the possible existence of a sympathetic control of cerebral blood flow. KEY WORDS microapplication technique norepinephrine cerebral arteriolar resistance vascular smooth muscle local arteriolar reactions bicarbonate ions• The influence of intravascular injections of norepinephrine (1-6) and the effects of cervical sympathetic nerve stimulation (7-17) have been frequently studied. The results, however,have not yet yielded a clear picture of the role of catecholamines in the control of cerebral blood flow. Several authors (1-3) have observed a decrease in cerebral blood flow following intravenous infusion of norepinephrine. However, this decrease might not be caused by the vasoconstrictor action of the infused norepinephrine on the cerebral vessels, but rather it might be due to the concomitant decrease in arterial CO 2 content during the course of

The effect of perivascular osmolarity on the diameter of pial arteries was studied in cats by the microapplication technique. Between 251 and 360 mosmoles/ liter, concentration-response curves were obtained for single vessels. Constriction occurred when perivascular osmolarity was decreased below 317 mosmoles /liter, and dilation occurred at osmolarities above this value. The effect was the same whether the osmolarity was changed by addition of mannitol or NaCl. Reduction of sodium concentration from 156 to 133 mEq/liter at constant osmolarity did not affect arteriolar diameter, but greater reductions in sodium concentration induced vasoconstriction. The results indicate that the resistance of pial arteries can be influenced by local changes in perivascular osmolarity.KEY WORDS perivascular ions local arteriolar reactions vascular smooth muscle cerebral blood vessels local control of cerebral blood flow cerebral arteriolar resistance

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