Modelling t-tubule function in cardiac ventricular myocytes
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| Year of publication | 2007 |
| Type | Conference abstract |
| MU Faculty or unit | |
| Citation | |
| Description | Cardiac transverse (t-) tubules form a complex network of membrane invaginations in mammalian ventricular myocytes, also known as the transverse-axial tubular system (TATS). Biophysically realistic computer models have been used to investigate experimental ambiguities and aspects of TATS function not currently amenable to experimental investigation: 1. Fraction of cell membrane within the TATS: Detubulation of rat ventricular myocytes causes a ~32% decrease of membrane capacitance whereas optical measurements suggest ~56% of the cell membrane within the TATS. Analysis of factors that may account for this discrepancy, and calculation of the combinations of t-tubule radius, length and density that produce t-tubular membrane fractions of 32% or 56%, suggest that the true fraction is at the upper end of this range. 2. Electrical coupling between the surface and TATS membranes: It has long been speculated that voltage control within the TATS may be inadequate, allowing voltage escape. Analysis based on cable theory, and simulations using models in voltage and current clamp mode, show that voltage spread across both membranes and its equilibration due to their tight electrical coupling is very fast (within 20 microseconds); thus membrane voltage is almost homogeneous over the whole cell membrane during activity. 3. Ion diffusion within TATS: The complexity of the TATS appears to restrict ion diffusion in the TATS lumen. Simulations performed using a model of a single tubule suggest that variable diameter along the length of the tubule and ion buffers in the tubule lumen play a major role in the restricted diffusion and thus in the slowed ion exchange between the TATS lumen and extracellular space. 4. Ion concentration changes in TATS and their consequences: Recent detubulation experiments suggest that many trans-membrane ion flux pathways (for ICa, INaCa, INaK and others) are located predominantly within the TATS. Incorporation of this distribution, and restricted diffusion between the tubular and extracellular spaces, into rat and guinea pig models results in changes of tubular [Ca2+] and [K+] during activity, which depend on stimulation rate. These changes (particularly Ca2+ depletion) cause a significant decrease of intracellular Ca2+ load and hence Ca2+ transient amplitude. These data suggest that the TATS may play an important role in modulating cardiac cell function. |
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