Pial Arterial Response to Topical Verapamil in Acute Closed Cranial Windows in Rabbits

Direct effects of Rho-kinase inhibitor on pial microvessels in rabbits

PURPOSE

Rho-kinase inhibitor is widely used for prevention of cerebral vascular spasm. However, the cerebral pial vascular action of Rho-kinase inhibitor has not been investigated. We therefore evaluated the direct effects of Y-27632, a Rho-kinase inhibitor, on pial microvessels.

METHOD

Experiments were performed on anesthetized rabbits. A closed cranial window was used to visualize the pial microcirculation. After baseline hemodynamic and pial vascular measurements, the cranial window was superfused with four increasing concentrations of Y-27632 (10(-9), 10(-7), 10(-6), 10(-5) mol l(-1); n = 7) dissolved in artificial cerebrospinal fluid for 7 min each. We measured the diameters of pial vessels, mean arterial pressure (MAP), heart rate (HR), and rectal temperature at 7 min after application of each Y-27632 concentration.

RESULTS

MAP, HR, rectal temperature, arterial pH, PaCO2, PaO2, and plasma Na(+), K(+) and glucose concentrations did not change significantly during the experimental period. Y-27632 at 10(-9) to 10(-7) mol l(-1) did not produce any significant change in pial arterioles. Topical application of Y-27632 at 10(-6) and 10(-5) mol l(-1) produced pial large (8.4 ± 5.7 and 19.8 ± 12.7 %) and small (10.1 ± 8.5 and 18.1 ± 12.3 %) arterioles dilation. However, Y-27632 did not produce any change in pial large and small venules.

CONCLUSION

We evaluated the direct effects of Y-27632 on pial microvessels. Y-27632 dilates only pial arterioles in a concentration-dependent manner, and most at a concentration of 10(-5) mol l(-1). Y-27632 is a potent cerebral pial arteriolar dilator but is not a venular dilator.

Transient cerebral hypoperfusion enhances intraarterial carmustine deposition into brain tissue

We hypothesized that bolus injections of lipid soluble chemotherapeutic drugs during transient cerebral hypoperfusion could significantly boost regional drug delivery. In the first two groups of New Zealand White rabbits we measured brain tissue carmustine concentrations after intravenous infusion, intraarterial infusion with normal perfusion, and after intraarterial injections during transient cerebral hypoperfusion. In the third group of animals we assessed the safety of the technique by assessing electroencephalographic changes for 6 h after flow arrest carmustine administration and subsequent histological examination. The brain tissue carmustine concentrations were fivefold to sevenfold higher when the drug was injected during cerebral hypoperfusion compared to a conventional intracarotid infusion (68.4 +/- 24.5 vs. 14.2 +/- 8.3 microg/g, n = 5 each, respectively, P < 0.0001). The brain tissue carmustine concentrations (y) were a linear function of the bolus dose (x) injected during cerebral hypoperfusion, y = 10.4 x x - 21 (R = 0.84, P < 0.001). Stable EEGs were recorded several hours after flow arrest carmustine exposure and histological examinations did not reveal any gross evidence of cerebral injury. Transient cerebral hypoperfusion during intraarterial bolus injection of carmustine significantly increases drug delivery. Clinical techniques that decrease CBF, such as, transient arterial occlusion by balloon tipped catheters, hyperventilation, hypothermia, induced hypotension, or transient circulatory arrest, could enhance intraarterial drug delivery to the brain. We believe that the mechanisms for improved drug delivery is the decrease in drug dilution by reduced or absent blood flow, decreased protein binding and a longer time for high concentrations of free drugs to transit through the blood brain barrier.

Electrocerebral silence after intracarotid propofol injection is a function of transit time

BACKGROUND

We hypothesized that the duration of electrocerebral (electroencephalogram, EEG) silence after bolus injection of propofol, a highly lipid soluble anesthetic drug, during transient cerebral hypoperfusion, will be directly related to the time taken by the bolus of drug to transit the cerebral circulation.

METHODS

We randomly divided 24 New Zealand White rabbits into two propofol volume groups: 0.5 and 0.8 mL groups. In each group, 12 animals received two intracarotid injections of 1% propofol: the first injection was made under normal physiological conditions and the second injection during cerebral hypoperfusion produced by bilateral carotid occlusion and IV bolus injection of adenosine and esmolol. We determined the duration of electrocerebral silence and the transit time of propofol emulsion under both cerebral circulation conditions. The transit time was measured by videomicroscopy through an implanted cranial window.

RESULTS

Cerebral hypoperfusion increased transit time with both low (2.3 +/- 0.7 to 55.7 +/- 21.4 s, n = 12, P < 0.0001) and high (2.2 +/- 0.6 to 62.5 +/- 31 s, n = 12, P < 0.0001) bolus volumes. The duration of electrocerebral silence during cerebral hypoperfusion was a function of the transit time with low (electrocerebral silence s = 152 + 2.3 x transit time, n = 12, r = 0.73, P = 0.007) and high (electrocerebral silence s = 186 + 3.2 x transit time, n = 12, r = 0.68, P = 0.02) bolus volumes.

CONCLUSION

These results suggest that manipulation of the transit time of highly lipid-soluble drugs profoundly enhances the effect site delivery.