Eletrophysiological properties of L-type Ca2+ currents in isolated adult mouse ventricular myocytes

The Whole-cell configuration of the patch-clamp technique was used to analyze the electrophysiological characteristics of the L-type calcium current (Ica) in single ventricular myocytes from hearts of adult mice. In Tyrode solution, ICa activated at -30 mV peaked at 0mV, and reverted near +60 mV. At 0mV, the peak current density was -8.1 + 2.5 pA/pF (N = 14). In a Na+ - and K+ -free solution containing 12 muM tetrodotoxin, and 10 mM Ca2+ or Ba2+ as charge carrier, the current-voltage relationship and the voltage dependence of inactivation were shifted about 10 mV to more depolarized voltages. The maximum Ba2+ current was two-times greater than the maximum Ca2+ current. The voltage dependencies of steady-state activation and inactivation were determined within the range of -70 to +50 mV and fitted with Boltzmann relations. The Ca2+ current showed half-maximal activation at -9.94 + 3.86 mV (slope factor (k) = 5.9 + 0.68 mV) and half-maximal inactivation at -27.65 + 5.74 mV (k = 6.37 + 2.79 mV), while the Ba2+ current showed half-maximal activation at -0.35 + 2.43 mV (k = 6.0 + 0.84 mV) and half-maximal inactivation at -20.33 + 2.40 mV (k = 5.36 + 1.10 mV). The time course of recovery of Ba2+ current from inactivation could be described using a single exponential function with a time constant of 83.37 msec. The overlap of activation and inactivation curves suggests the existence of an L-type Ca2+ window current with a maximal amplitude near -20mV.

Effects of extracellular sodium, quinidine, and ryanodine on slow response excitability in rabbit atrial trabeculae

1. We investigated Na(+)-Ca2+ exchange and the involvement of the sarcoplasmic reticulum in frequency-dependent slow response excitability enhancement in rabbit atrial trabeculae. 2. Slow responses were induced in a modified Tyrode solution containing high K+ and Ba2+ and conventional electrophysiological techniques were used for stimulating and recording membrane potentials. 3. Under these conditions, the frequency-dependence of slow response excitability can be demonstrated with excitability enhancement as stimulation frequency is increased (0.25 to 1.0 Hz). 4. The frequency-dependent excitability enhancement depends on external Na+, increasing in high-[Na+]o (173.8 mM) and decreasing in low-[Na+]o (103.8 mM) media. 5. Quinidine (10 microM) and ryanodine (10 microM) decrease frequency-dependent slow response excitability enhancement. 6. These results indicate that the Na(+)-Ca2+ exchange might have an important role in frequency-dependent excitability enhancement of slow responses. Moreover, we suggest that the control of internal Ca2+ by the sarcoplasmic reticulum might have an additional role in regulating the excitability enhancement process in depolarized atrial trabeculae

Frequency-dependent excitability enhancement in isolated ventricular myocardial cells

The understanding of the mechanisms underling the frquency-dependent slow response ecitability enhancement has been hndered by the problem inhyerent in multicellular preparations. These include ion acdcumulation/depletion in intercellular space and difficulties in the spatial control of transmembrane voltage. In the present communication we show that isolated ventricular cells exposed to a depolarizing (high potassium-barium containing) solution present electrophysiological properties similar to those of mulcellular preparations: stable resting potential of -45.2 ñ 0.7 mV (mean ñ SEM, N = 57) in 75% of the cells and spontaneous activity in the remaining 25% (maximum diatolic potential of -41.9 ñ 1.2 mV, N=19)ñ high input resistance and slow response, under current clamp conditions. Under whole cell voltage clamp conditions with -45 mV holding potential, transient outward and delayed potassium currents as well as typical L type calcium channel are present. These cells also present thye frequency-dependent excitability enhancement of the slow response, with the threshold stimulus at 1 Hz corresponding to about 50% of that obtained at 0.1 Hz. Thus, isolated ventricular cells constitute a suitable model for the study of frequency-dependent exitability enhancement of the slow response