Automaticity and conduction properties of bio-artificial pacemakers assessed in an in vitro monolayer model of neonatal rat ventricular myocytes.

AIMS: A better understanding of the ionic mechanisms for cardiac automaticity can lead to better strategies for engineering bio-artificial pacemakers. Here, we attempted to better define the relative contribution of I(f) and I(K1) in the generation of spontaneous action potentials (SAPs) in cardiomyocytes (CMs). METHODS AND
RESULTS: Monolayers of neonatal rat ventricular myocytes (NRVMs) were transduced with a recombinant adenovirus (Ad) to express a gating-engineered HCN1 construct (HCN1-DeltaDeltaDelta) for patch-clamp and multielectrode array (MEA) recordings. Single NRVMs exhibited a bi-phasic response in the generation of SAPs (62.6 +/- 17.4 b.p.m., Days 1-2; 194.3 +/- 12.3 b.p.m., Days 3-4; 73% quiescent, Days 9-10). Although automaticity time-dependently decreased and subsequently ceased, I(f) remained fairly stable (-5.2 +/- 1.1 pA/pF, Days 1-2; -5.1 +/- 1.4 pA/pF, Days 7-8; -4.3 +/- 1.3 pA/pF, Days 13-14). In contrast, I(K1) declined rapidly (from -16.9 +/- 2.7 pA/pF on Days 1-2 to -4.4 +/- 1.6 pA/pF on Days 5-6). Maximum diastolic potential/resting membrane potential (r = 0.89) and action potential duration at 50% (APD(50), r = 0.73) and 90% (APD(90), r = 0.75) but not the firing rate (r = -0.3) were positively correlated to the I(K1). Similarly, monolayer NRVMs ceased to spontaneously fire after long-term culture. Ad-HCN1-DeltaDeltaDelta transduction restored pacing in silenced individual and monolayer NRVMs but with reduced conduction velocity and field potential amplitude.
CONCLUSION: We conclude that the combination of I(K1) and I(f) primes CMs for bio-artificial pacing by determining the threshold. However, I(f) functions as a membrane potential oscillator to determine the basal firing frequency. Future engineering of automaticity in the multicellular setting needs to have conduction taken into consideration.