Modelling the Membrane Potential Dependence on Non-Specific Cation Channels in Canine Articular Chondrocytes

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In a previous report, we showed that the predominant ion channel in potassium-free solutions was a gadolinium III (Gd3+) sensitive non-specific cation channel, with functional characteristics similar to that expected for transient receptor potential (TRP) type channels.In this study reverse transcription-PCR (RT-PCR) was used to investigate the expression of TRP channels in canine articular chondrocytes. Both a mathematical model based on the Goldman-Hodgkin-Katz voltage equation and current clamp whole-cell electrophysiology were then used to investigate the effect of these channels on membrane potential (Vm).Chondrocytes isolated from canine articular cartilage were cultured for 5 days in Dulbeccos Modified Eagles Medium with 10% Foetal Calf Serum. For RT-PCR analysis, total RNA was extracted from first passage cells. Electrophysiological recording was carried out on first to third passage cells.RT-PCR analysis of chondrocyte mRNA, and subsequent sequencing of products, showed a member of the TRP vanilloid group of channels (TRPV4) to be present; sequence homology to the human TRPV4 was 94%. We have so far failed to find mRNA for the functionally similar TRPC3 and TRPC6 channels.Using whole-cell and single-channel data from our own experiments and the literature, our model predicts the membrane to be heavily dependent on the activity of TRPV4. Simulated block of all non-specific cation channels in the chondrocyte membrane leads to a predicted −27mV change in Vm. This prediction closely matches our current clamp experiments with 100μM Gd3+ inducing a −32 ± 1mV (n = 6) change of Vm.Previously, large-conductance chloride and voltage-sensitive potassium channels have been reported to be important in the maintenance of chondrocyte Vm. The data presented here showsthat the TRPV4 channel also has a significant contribution to maintenance of the chondrocyte Vm.