Abstract
1. Pericytes, which are contractile cells located on the outer wall of microvessels, are thought to be particularly important in the retina where the ratio of these cells to vascular endothelial cells is the highest of any tissue. Retinal pericytes are of interest since they may regulate capillary blood flow and because their selective loss is an early event in diabetic retinopathy, which is a common sight-threatening disorder associated with dysfunction of the blood-retinal barrier. 2. Although a breakdown in the vascular endothelial barrier is a frequent pathophysiological event, knowledge of the effects of blood-derived molecules on pericyte function is limited. Based on the premise that ion channels play a vital role in cellular function, we examined the effect of serum on the ionic currents of retinal pericytes. To do this, we used the perforated-patch configuration of the patch-clamp technique to monitor the whole-cell currents of pericytes located on freshly isolated rat retinal microvessels. 3. Exposure to serum reversibly activated inward and outward currents in virtually all of the sampled retinal pericytes. Two types of sustained conductances were induced by serum. These were a calcium-permeable non-specific cation (NSC) current and a voltage-dependent potassium current. In addition, exposure to serum increased the activity of chloride channels which caused transient depolarizing currents. 4. Associated with the activation of these conductances, the membrane potential showed a sustained decrease of 10 +/- 2 mV from -56 mV to -46 mV and, also, transient depolarizations to near -30 mV. The serum-induced depolarizations can activate the voltage-gated calcium channels expressed by the retinal pericytes. 5. Calcium-permeable NSC channels appear to play a critical role in the response of pericytes to serum-derived molecules. Consistent with this, activation of the chloride and potassium channels was sensitive to SK&F 96365, which is a blocker of NSC channels. In addition, chloride and potassium channel activation was dependent on extracellular calcium. 6. The effects of serum on the activity of channels in retinal pericytes were qualitatively mimicked by insulin-like growth factor-1 (IGF-1), which is a normal constituent of the blood. 7. There are significant differences in the effects of serum on retinal pericytes compared with vascular smooth muscle cells. Serum activated sustained conductances in retinal pericytes but not in the vascular smooth muscle cells. This suggests a fundamental difference in the mechanisms by which serum-derived molecules affect these two types of cells. 8. We conclude that serum-derived molecules, such as IGF-1, can activate several types of ion channels in retinal pericytes. These changes in channel activity are likely to influence pericyte function at sites of a breakdown in the blood-retinal barrier.