Purchase this article with an account.
Jeffrey W. Kiel, William J. Lavery; Effect of Alpha Adrenergic Blockade on the Rabbit Ocular Hydrodynamic Response to Tilt. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1521.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Changing from an upright to a supine position causes gravity-dependent increases in cranial arterial and venous pressure. In humans and animals, the IOP increase to this posture change is smaller than expected for the change in the height of the hydrostatic column between the eye and the heart. This implies the action of a venous regulatory mechanism since the Goldmann equation predicts that IOP and episcleral venous pressure should change equally. Anatomical studies show adrenergic innvervation of the episcleral circulation. The purpose of this study was to assess the effect of an alpha adrenergic antagonist on the ocular hydrodynamic response to tilt in a rabbit model.
In anesthetized rabbits (n=11), we measured mean arterial pressure (MAP), IOP, and orbital venous pressure (OVP) by direct cannulation; carotid blood flow (BFcar) by transit time ultrasound, heart rate (HR) by a digital cardiotachometer, and aqueous flow (Flow) by fluorophotometry. The protocol entailed 60 min of baseline measurements in a head up position, followed by 90 min of tilt (i.e., hindquarters level with head), then phentolamine (0.1 mg/kg, iv) was given and measurements continued for another 90 min of tilt. Results were analyzed by repeated measures ANOVA and Newman-Keuls post-hoc testing. p<0.05 was considered significant.
Tilt caused immediate, significant increases in MAP (5.0 ± 0.9 mmHg), IOP (1.0 ±0.3 mmHg), and OVP (1.5 ± 0.1 mmHg). BFcar, HR and Flow were unaffected. The increase in MAP was significantly greater than the increases in IOP or OVP. Phentolamine decreased MAP, IOP, and Flow, while BFcar, HR and OVP were unchanged.
The decrease in Flow accounts for the decrease in IOP after phentolamine. That result, coupled with the lack of OVP response to phentolamine, indicates that alpha adrenergic tone does not contribute significantly to venous pressure regulation during tilt. The greater effect of gravity on cranial arterial pressure than venous pressure requires further study.
This PDF is available to Subscribers Only