Simulations of reduced conduction reserve in the diabetic rat heart: response to uncoupling and reduced excitability.

Experimental results have shown that action potential (AP) conduction in ventricular tissue from streptozotocin-diabetic (STZ) rats is compromised. This was manifest as increased sensitivity of conduction velocity (CV) to the gap junction uncoupler heptanol, as well as increased sensitivity of CV to reduced cellular excitability due to elevated extracellular K(+) concentration, in the STZ hearts. This "reduced conduction reserve" has been suggested to be due to lateralization of connexin43 (Cx43) proteins, rendering them nonfunctional, resulting in compromised intercellular electrical coupling. In this study, we have used computer simulations of one-dimensional AP conduction in a model of rat ventricular myocytes to verify this interpretation. Our results show that compromised intercellular coupling indeed reduces conduction reserve and predict a response to gap junction uncoupling with heptanol that is consistent with experiments. However, our simulations also show that compromised intercellular coupling is insufficient to explain the increased sensitivity to reduced cellular excitability. A thorough investigation of possible underlying mechanisms, suggests that subtle alterations in the voltage-dependence of steady-state gating for the Na(+) current (I (Na)), combined with compromised intercellular coupling, is a likely mechanism for these observations.