Effects of Lactate on the Physiology of Pericyte–Containing Retinal Microvessels

Abstract

Abstract: : Purpose: Unlike most tissues, lactate is produced by retinal cells under physiological conditions. One cellular source for this molecule is the glial, which may use lactate as a signal to regulate retinal blood flow. Our goal was to elucidate how lactate affects the physiology of pericyte–containing retinal microvessels, which are likely to play a role in the regulation of local vascular perfusion. Methods: Complexes of microvessels were isolated from papain–treated rat retinas; experiments were done within 3 h; osmolarity and pH of the bathing solutions were carefully controlled. Ionic currents were monitored in pericytes via perforated–patch pipettes. Changes in pericyte contractility were visualized with the aid of differential interference optics and time–lapse photography. Results: Exposure of freshly isolated retinal microvessels to lactate caused pericytes to reversibly hyperpolarize in a concentration–dependent manner; the membrane potential of sampled pericytes increased from –38 ± 1 mV to –47 ± 2 mV (P < 0.001, n = 20) in the presence of 20 mM lactate. The lactate–induced hyperpolarization was generated by an outward current that was reversibly blocked by ouabain (500 µM), a Na⁺/K⁺ pump inhibitor. This Na⁺/K⁺ pump activation appeared to be dependent upon Na⁺/H⁺ exchangers because dimethylamiloride (DMA, 30 µM), an inhibitor of these exchangers, reduced the amplitude of the lactate–induced outward current by 86 ± 10%. These observations are consistent with lactate being transported via a proton–coupled carrier, Na⁺/H⁺ exchangers being activated to regulate [H⁺]_i and Na⁺/K⁺ pumps responding to the Na⁺ influx via Na⁺/H⁺ exchangers. In other experiments, time–lapse photography revealed that pericytes contracted when microvessels were exposed to lactate. Consistent with the Na⁺/H⁺ exchanger playing a critical role in the contractile response of pericytes to lactate, exposure of microvessels to DMA (30 µM) switched lactate–induced contractions into relaxations. Conclusions: Lactate, which may serve as a glial–to–vascular signal in the retina, regulates pericyte physiology by mechanisms involving the activation of the microvascular Na⁺/H⁺ exchanger.

Keywords: vascular cells • blood supply • retina