Effects of cyclic intraocular pressure on conventional outflow facility.

PURPOSE: In vivo, biomechanical stress plays an important role in tissue physiology and pathology, affecting cell and tissue behavior. Even though conventional outflow tissues are constantly exposed to dynamic changes in intraocular pressure, effects of such biomechanical stressors on outflow function have not been analyzed. The purpose of the present study was to determine the effect(s) of ocular pulse on conventional outflow facility in perfused anterior segments. METHODS: The anterior segment perfusion model was used to investigate the impact of ocular pulsation on human and porcine outflow facility. To determine tissue viability of human anterior segments, three complementary techniques (postperfusion morphology and cell density of outflow tissues plus central corneal thickness measurements over time of perfusion) were used. RESULTS: A consistent decrease in outflow facility was observed in response to cyclic intraocular pressure in both porcine (-29.96% +/- 6.56; P = 0.009) and human (-27.65% +/- 8.26; P = 0.010) perfused anterior segments. Viability data showed no significant difference between control and experimental anterior segments, with respect to postperfusion histologic evaluations (P = 0.227) or change in central corneal thickness over time (P = 0.289). In contrast, the cellularity of the trabecular meshwork in experimental (cyclically pulsed) anterior segments (333.86 +/- 22.15 nuclei/field of view) was greater than in the control eyes (290.47 +/- 17.60, P = 0.05). CONCLUSIONS: Decreased outflow facility in cyclically pulsed anterior segments is not a function of cell or tissue damage, but rather is an active response of the conventional outflow tissues to a biomechanical stimulus. In fact, the observation of increased cellularity in tissues exposed to cyclic stress suggests a physiological benefit of mechanical stress to outflow cells in organ culture.

Schlemm's canal endothelia, lymphatic, or blood vasculature?

In the human eye, the final barrier for aqueous humor to cross before returning to systemic circulation is the inner wall of Schlemm's canal. Unfortunately, the specific contribution of the inner wall to total outflow resistance in the conventional pathway is unknown in both normal and glaucomatous eyes. To better understand inner wall physiology, we contrasted it with 2 specialized continuous endothelia, initial lymphatic, and blood capillary endothelia. Specifically, we compare their developmental origin, morphology, junctional complexes, microenvironment, and physiologic responses to different biomechanical factors. Our evaluation concludes that the inner wall of Schlemm's canal is unique, sharing extraordinary characteristics with both types of specialized endothelia in addition to having distinctive features of its own.

A Novel Approach to Improving Fat Delivery in Neonatal Enteral Feeding.

Continuous infusion systems used for enteral nutrition support in the neonatal intensive care unit deliver as little as 60% of the fat in human milk to the neonate. This study determined the effect of mixing common feedings for preterm infants in the feeding bag and tubing on fat losses during enteral feeding. Laboratory models were developed to assess the contribution of various mixing techniques to delivered fat content. Fat content was measured periodically during feeding and compared to baseline measurements. A multistage approach incorporating a feeding bag inverter and a tubing circulation loop delivered >90% of milk fat when used in conjunction with a commercial continuous infusion system. With unfortified human milk, this approach delivered 91.9% ± 1.5% of fat content over a one hour feed, significantly greater (p < 0.01) than 77.5% ± 2.2% delivered by continuous infusion controls (Mean ± SEM). With fortified human milk, this approach delivered 92.1% ± 2.4% of fat content, significantly greater (p < 0.01) than 79.4% ± 1.0% delivered by a non-adapted infusion system (Mean ± SEM). Mixing human milk during continuous infusion improves fat delivery, which may improve nutrition and growth outcomes in low birth weight neonates.

Cyclic mechanical stress and trabecular meshwork cell contractility.

PURPOSE: Ocular pulse decreases outflow facility of perfused anterior segments. However, the mechanism by which conventional outflow tissues respond to cyclic intraocular pressure oscillations is unknown. The purpose of the present study was to examine responses of trabecular meshwork (TM) cells to cyclic biomechanical stress in the presence and absence of compounds known to affect cell contractility. METHODS: To model flow in the juxtacanalicular region of the TM and to measure changes in transendothelial flow, human TM cell monolayers on permeable filters were perfused at a constant flow rate until reaching a stable baseline pressure and then were exposed to cyclic stress with an average amplitude of 2.7 mm Hg peak to peak at a 1-Hz frequency for 2 hours in the presence or absence of compounds known to affect cell contractility (isoproterenol, Y27632, pilocarpine, and nifedipine). Pressure was recorded continuously. Immunocytochemistry staining was used to determine filamentous actin stress fiber content, whereas Western blot analysis was used to measure the extent of myosin light chain (p-MLC) phosphorylation and ratio of filamentous to globular actin. RESULTS: Human TM cells respond to cyclic pressure oscillations by increasing mean intrachamber pressure (decreasing hydraulic conductivity) (126.13% +/- 2.4%; P < 0.05), a response blocked in the presence of Y27632, a rho-kinase inhibitor (101.35 +/- 0.59; P = 0.234), but not isoproterenol, pilocarpine, or nifedipine. Although mechanical stress appeared to have no effect, Y27632 decreased phosphorylated myosin light chain, filamentous/globular actin ratio, and stress fiber formation in TM cells. CONCLUSIONS: Human TM cells respond to cyclic mechanical stress by increasing intrachamber pressure. Pulse-mediated effects are blocked by Y27632, implicating a role for Rho-kinase-mediated signaling and cellular contractility in ocular pulse-associated changes in outflow facility.