David O. Zamora

Episcleral venous pressure responses to topical nitroprusside and N-nitro-L-arginine methyl ester

PURPOSE To determine the episcleral venous pressure (EVP) responses to nitroprusside (NP) and L-NAME. METHODS In anesthetized rabbits (n = 36), arterial pressure and IOP were measured by direct cannulation, and carotid blood flow and heart rate were measured with an ultrasound flowmeter and cardiotachometer. EVP was measured in two groups with a servonull system. Group 1 (n = 13) was given NP (50 microL, 10 mg/mL). Group 2 (n = 10) was given L-NAME (100 microL, 10 mg/mL) followed by NP (50 microL, 10 mg/mL). In group 3 (n = 13), fluorophotometric aqueous flow was measured before and after NP (100 microL, 10 mg/mL). RESULTS Systemic parameters were unaffected by treatment in all groups. In group 1, NP increased EVP from 9.1 +/- 0.6 to 11.6 +/- 0.8 mm Hg (P < 0.01) and IOP from 18.7 +/- 1.4 to 23.9 +/- 1.6 mm Hg (P < 0.01). In group 2, L-NAME lowered EVP from 11.5 +/- 1.2 to 8.8 +/- 1.0 mm Hg (P < 0.01) and subsequent NP increased EVP to 13.9 +/- 1.7 mm Hg (P < 0.01 versus L-NAME and baseline). L-NAME decreased IOP from 20.8 +/- 1.7 to 16.7 +/- 1.8 mm Hg (P < 0.01), and then it increased to 20.7 +/- 1.3 mm Hg after NP (P < 0.01 versus L-NAME and P > 0.05 versus baseline). In group 3, NP increased IOP from 16.6 +/- 0.7 to 20.0 +/- 0.9 mm Hg (P < 0.01) but did not alter aqueous flow (2.65 +/- 0.3 vs. 3.0 +/- 0.3 microL/min, P > 0.05). CONCLUSIONS Because a topical NO donor raises EVP and a topical NO synthase inhibitor lowers EVP, the authors conclude that EVP is modulated by NO.

Topical proparacaine and episcleral venous pressure in the rabbit.

PURPOSE To determine the effect of proparacaine-induced topical anesthesia on episcleral venous pressure (EVP). METHODS In anesthetized rabbits (n = 11), EVP was measured with a servonull micropressure system, with glass pipettes with 2- to 3-microm tips used to cannulate episcleral veins. Additional measurements included arterial, intraocular, and orbital venous pressures obtained by direct cannulation, to assess the ocular pressure gradients, and carotid blood flow and heart rate, to verify preparation stability. The protocol entailed 5 to 10 minutes of stable baseline recording followed by topical application of proparacaine (0.5%, 10 microL) with continued measurements for another 5 to 15 minutes. RESULTS Baseline EVP without topical anesthesia was 12.3 +/- 1.1 mm Hg. EVP decreased significantly to 8.7 +/- 0.9 mm Hg within minutes after application of proparacaine. A small decrease also occurred in intraocular pressure. All other measured variables were unchanged. CONCLUSIONS These results suggest that the episcleral circulation is under tonic neural control and that either an upstream resistance site is under tonic vasodilatory control or a downstream site is under vasoconstrictor control.