Simultaneous monitoring of cellular depolarization and contraction: a new method to investigate excitability and contractility in isolated smooth muscle cells

The present experiments were performed to establish a method for simultaneous monitoring of excitation and contraction in isolated smooth muscle cells. The smooth muscle cells were dissociated from the colons of Wistar rats by enzymatic digestion. All the experiments were performed on mixtures of circular and longitudinal cells. In a first set of experiments, focal extracellular potentials (FEPs) and transmembrane action potentials (APs) were simultaneously recorded from the cells by use of extracellular and intracellular pipettes, respectively. In a second set of experiments, cellular contraction induced by chemical stimulation was monitored simultaneous with the FEP recordings. The FEPs had spike and plateau amplitudes of 44.5 ± 2.3 and 8.9 ± 0.7 mV, respectively, and reproduced the general morphology of gastrointestinal APs. The parallel mechanical measurements from the rat colonic cells showed that they shortened with an average peak contraction of 8.8 ± 1.4 μm and an average contraction velocity of 8.2 ± 0.9 μm/s, to develop an average peak force of 1.2 ± 0.2 μN, and generated an average peak power of 36 ± 15 pW. Simultaneous monitoring of FEPs and cellular contraction demonstrates correlations between the electrical and mechanical events taking place in the investigated cells.

Monophasic action potentials generated by bidomain modeling as a tool for detecting cardiac repolarization times

Unipolar electrograms (EGs) and hybrid (or unorthodox or unipolar) monophasic action potentials (HMAPs) are currently the only proposed extracellular electrical recording techniques for obtaining cardiac recovery maps with high spatial resolution in exposed and isolated hearts. Estimates of the repolarization times from the HMAP downstroke phase have been the subject of recent controversies. The goal of this paper is to computationally address the controversies concerning the HMAP information content, in particular the reliability of estimating the repolarization time from the HMAP downstroke phase. Three-dimensional numerical simulations were performed by using the anisotropic bidomain model with a region of short action potential durations. EGs, transmembrane action potentials (TAPs), and HMAPs elicited by an epicardial stimulation close or away from a permanently depolarized site were computed. The repolarization time was computed as the moment of EG fastest upstroke (RTeg) during the T wave, of HMAP fastest downstroke (RTHMAP), and of TAP fastest downstroke (RTtap). The latter was taken as the gold standard for repolarization time. We also compared the times (RT90HMAP, RT90tap) when the HMAP and TAP first reach 90% of their resting value during the downstroke. For all explored sites, the HMAP downstroke closely followed the TAP downstroke, which is the expression of local repolarization activity. Results show that HMAP and TAP markers are highly correlated, and both markers RTHMAP and RTeg (RT90HMAP) are reliable estimates of the TAP reference marker RTtap (RT90tap). Therefore, the downstroke phase of the HMAP contains valuable information for assessing repolarization times.

The Gurvich waveform has lower defibrillation threshold than the rectilinear waveform and the truncated exponential waveform in the rabbit heart

Implantable cardioverter defibrillator studies have established the superiority of biphasic waveforms over monophasic waveforms. However, external defibrillator studies of biphasic waveforms are not as widespread. Our objective was to compare the defibrillation efficacy of clinically used biphasic waveforms, i.e., truncated exponential, rectilinear, and quasi-sinusoidal (Gurvich) waveforms in a fibrillating heart model. Langendorff-perfused rabbit hearts (n = 10) were stained with a voltage-sensitive fluorescent dye, Di-4-ANEPPS. Transmembrane action potentials were optically mapped from the anterior epicardium. We found that the Gurvich waveform was significantly superior (p < 0.05) to the rectilinear and truncated exponential waveforms. The defibrillation thresholds (mean ± SE) were as follows: Gurvich, 0.25 ± 0.01 J; rectilinear-1, 0.34 ± 0.01 J; rectilinear-2, 0.33 ± 0.01 J; and truncated exponential, 0.32 ± 0.02 J. Using optically recorded transmembrane responses, we determined the shock-response transfer function, which allowed us to predict the cellular response to waveforms at high accuracy. The passive parallel resistor-capacitor model (RC-model) predicted polarization superiority of the Gurvich waveform in the myocardium with a membrane time constant (τm) of less than 2 ms. The finding of a lower defibrillation threshold with the Gurvich waveform in an in vitro model of external defibrillation suggests that the Gurvich waveform may be important for future external defibrillator designs.Key words: defibrillation, optical mapping, biphasic waveform, Gurvich waveform.

Ca(2+)-dependent outward currents in myocytes from epicardial border zone of 5-day infarcted canine heart

Myocytes from the epicardial border zone (EBZ) of the 5-day infarcted canine heart (IZ) have abnormal transmembrane action potentials, reduced L-type Ca2+ currents (ICa,L) and altered intracellular Ca2+ (Cai) transients compared with those of normal epicardial myocytes (NZ). We hypothesized that altered Cai cycling might be reflected in differences in Cai-dependent outward currents (Ito2). We recorded Ito2 in NZ and IZ using whole cell patch-clamp techniques. Ito2 was defined as the amplitude of the 4-aminopyridine-resistant transient outward current that was blocked by 200 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or DIDS+ ryanodine (2 microM). Ito2 were present in both NZ and IZ, but peak density was significantly reduced in IZ, particularly at positive plateau voltages. Time course of decay of Ito2 was biexponential and similar in NZ and IZ. A given peak ICa,L was usually associated with a smaller peak Ito2 in IZ. These differences were exaggerated when Ito2 and Cai transients were determined in rapidly paced cells. In summary, myocytes surviving in the EBZ of the infarcted heart have Ito2, yet they are reduced in density and can vary, particularly at fast pacing rates.