Diabetes-Induced Inhibition of Voltage-Dependent Calcium Channels in the Retinal Microvasculature: Role of Spermine

Abstract

PURPOSE.Although decentralized control of blood flow is particularly important in the retina, knowledge of the functional organization of the retinal microvasculature is limited. Here, the authors characterized the distribution and regulation of L-type voltage-dependent calcium channels (VDCCs) within the most decentralized operational complex of the retinal vasculature-the feeder vessel/capillary unit-which consists of a capillary network plus the vessel linking it with a myocyteencircled arteriole. METHODS. Perforated-patch recordings, calcium-imaging, and time-lapse photography were used to assess VDCC-dependent changes in ionic currents, intracellular calcium, abluminal cell contractility, and lumen diameter, in microvascular complexes freshly isolated from the rat retina. RESULTS. Topographical heterogeneity was found in the distribution of functional VDCCs; VDCC activity was markedly greater in feeder vessels than in capillaries. Experiments showed that this topographical distribution occurs, in large part, because of the inhibition of capillary VDCCs by a mechanism dependent on the endogenous polyamine spermine. An operational consequence of functional VDCCs predominantly located in the feeder vessels is that voltage-driven vasomotor responses are generated chiefly in this portion of the feeder vessel/capillary unit. However, early in the course of diabetes, this ability to generate voltage-driven vasomotor responses becomes profoundly impaired because of the inhibition of feeder vessel VDCCs by a spermine-dependent mechanism. CONCLUSIONS. The regulation of VDCCs by endogenous spermine not only plays a critical role in establishing the physiological organization of the feeder vessel/capillary unit, but also may contribute to dysfunction of this decentralized operational unit in the diabetic retina. (Invest Ophthalmol Vis Sci.

Calcium Signalling in Pericytes

“…Electrophysiological and pharmacological studies have revealed that pericytes are electrically excitable cells [10,43,44,45]. The resting membrane potential of pericytes ranges from -32 to -70 mV [43,45] and can be modulated by different ion channels present in their plasma membrane such as Ca 2+ -activated Cl - channels (Cl Ca ) [18,46], Ca 2+ -activated K + channels (K Ca ) [45], ATP-sensitive K + channels (K ATP ) [47], inwardly rectifying K + channels (K IR ) [43,45], voltage-dependent K + channels (K V ) [45], voltage-gated Na + channels [48], nonselective cation channels [21] and transient receptor potential channels [49].…”

“…The resting membrane potential of pericytes ranges from -32 to -70 mV [43,45] and can be modulated by different ion channels present in their plasma membrane such as Ca 2+ -activated Cl - channels (Cl Ca ) [18,46], Ca 2+ -activated K + channels (K Ca ) [45], ATP-sensitive K + channels (K ATP ) [47], inwardly rectifying K + channels (K IR ) [43,45], voltage-dependent K + channels (K V ) [45], voltage-gated Na + channels [48], nonselective cation channels [21] and transient receptor potential channels [49]. Voltage-clamp experiments performed on pericytes in culture [10] or on isolated microvessels [10,44,45] show that pericytes contain L-type voltage-gated Ca 2+ channels (VGCC). However, comparative studies performed on the freshly isolated retinal microvascular complexes [44] and kidney tissue slices [45] show that VGCC-mediated changes in ionic currents are markedly (i.e.…”

“…Voltage-clamp experiments performed on pericytes in culture [10] or on isolated microvessels [10,44,45] show that pericytes contain L-type voltage-gated Ca 2+ channels (VGCC). However, comparative studies performed on the freshly isolated retinal microvascular complexes [44] and kidney tissue slices [45] show that VGCC-mediated changes in ionic currents are markedly (i.e. 7.5 times) greater in myocytes than in pericytes [44,45].…”

“…However, comparative studies performed on the freshly isolated retinal microvascular complexes [44] and kidney tissue slices [45] show that VGCC-mediated changes in ionic currents are markedly (i.e. 7.5 times) greater in myocytes than in pericytes [44,45]. These findings indicate that potential-dependent, dihydropyridine-sensitive VGCCs play a significant role in excitation-contraction coupling in arteriolar myocytes but may either be poorly distributed or physiologically silent in pericytes.…”

“…It is not known whether molecular isoforms of VGCC are expressed in pericytes. In rat retinal pericytes, this difference in VGCC function is, in large part, due to the selective inhibition of the VGCCs in the pericytes by endogenous spermine, a polyamine known to inhibit L-type calcium channels [44]. …”