Whole-cell patch clamp experiments were carried out in rat striatal brain slices. In a subset of striatal neurons (70-80%), NMDA-induced inward currents were inhibited by the adenosine A2A receptor selective agonist CGS 21680. The non-selective adenosine receptor antagonist 8-(p-sulphophenyl)-theophylline and the A2A receptor selective antagonist 8-(3-chlorostyryl)caffeine abolished the inhibitory action of CGS 21680. Intracellular GDP-beta-S, which is known to prevent G protein-mediated reactions, also eliminated the effect of CGS 21680. Extracellular dibutyryl cAMP, a membrane permeable analogue of cAMP, and intracellular Sp-cAMPS, an activator of cAMP-dependent protein kinases (PKA), both abolished the CGS 21680-induced inhibition. By contrast, Rp-cAMPS and PKI 14-24 amide, two inhibitors of PKA had no effect. Intracellular U-73122 (a phospholipase C inhibitor) and heparin (an inositoltriphosphate antagonist) prevented the effect of CGS 21680. Finally, a more efficient buffering of intracellular Ca2+ by a substitution of EGTA (11 mM) by BAPTA (5.5 mM) acted like U-73122 or heparin. Hence, A2A receptors appear to negatively modulate NMDA receptor channel conductance via the phospholipase C/inositoltriphosphate/Ca2+ pathway rather than the adenylate cyclase/PKA pathway.

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Ion channels were studied using the whole-cell patch clamp technique in bovine retinal and choroidal microvascular endothelial cells (MVEC) cultured under the same conditions. The two types of MVEC expressed inward currents at hyperpolarizing voltage steps and showed small outward currents at depolarizing steps. The extrapolated reversal potentials of the inward currents were near to the potassium equilibrium potential. Cs⁺ and the K⁺ channel blocker TEA reduced the amplitudes of the currents indicating the selectivity and permeability for potassium. This was confirmed by changes of outside K⁺ concentration shifting the I-V curves to the right. RT-PCR studies revealed the presence of mRNA of Kir2.1, an inwardly rectifying K⁺ channel, in retinal and choroidal MVEC. The profile of the small outward currents is related to the Kv family but not identical with the Kv1.4 subtype.

We have recently described cytokeratin-positive (CK(+)) and cytokeratin-negative (CK(-)) microvascular endothelial cells (MVECs) in the bovine corpus luteum. The two phenotypes show a different expression and release of adhesion molecules and cytokines. Since secretion of mediators is specifically regulated by the electrophysiological membrane parameters, this report will compare voltage-dependent potassium (K(+)) channels in the two cell types cultured under the same conditions. CK(+) and CK(-) MVECs derived from the microvascular bed of one organ differ in their K(+)-channel characteristics. In CK(-) MVECs, an inwardly rectifying K(+) current was discovered, showing the characteristics of the Kir2.1. CK(+) MVECs displayed a voltage-dependent K(+) current that activates rapidly on depolarization and inactivates very slowly, and is associated with a member of the Kv family. The mRNA for Kir2.1 was identified by RT-PCR in CK(-) and CK(+) MVECs, but there was no evidence of Kv1.4 mRNA in either of them. The function of Kir2.1 in CK(-) MVECs might be induced during cultivation, whereas CK(+) MVECs appear to be more resistant to environmental conditions and do not express an inward current.

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