Effect of temperature on isoprenaline- and barium-induced slow action potentials in guinea-pig ventricular strips

Cardiac tissue slices: preparation, handling, and successful optical mapping

Cardiac tissue slices are becoming increasingly popular as a model system for cardiac electrophysiology and pharmacology research and development. Here, we describe in detail the preparation, handling, and optical mapping of transmembrane potential and intracellular free calcium concentration transients (CaT) in ventricular tissue slices from guinea pigs and rabbits. Slices cut in the epicardium-tangential plane contained well-aligned in-slice myocardial cell strands (“fibers”) in subepicardial and midmyocardial sections. Cut with a high-precision slow-advancing microtome at a thickness of 350 to 400 μm, tissue slices preserved essential action potential (AP) properties of the precutting Langendorff-perfused heart. We identified the need for a postcutting recovery period of 36 min (guinea pig) and 63 min (rabbit) to reach 97.5% of final steady-state values for AP duration (APD) (identified by exponential fitting). There was no significant difference between the postcutting recovery dynamics in slices obtained using 2,3-butanedione 2-monoxime or blebistatin as electromechanical uncouplers during the cutting process. A rapid increase in APD, seen after cutting, was caused by exposure to ice-cold solution during the slicing procedure, not by tissue injury, differences in uncouplers, or pH-buffers (bicarbonate; HEPES). To characterize intrinsic patterns of CaT, AP, and conduction, a combination of multipoint and field stimulation should be used to avoid misinterpretation based on source-sink effects. In summary, we describe in detail the preparation, mapping, and data analysis approaches for reproducible cardiac tissue slice-based investigations into AP and CaT dynamics.

Differential temperature-dependency of electrophysiological and inotropic actions of nifedipine, verapamil and cinnarizine in K+-depolarized ventricular myocardium

1. Temperature dependency (in the range 27-37 degrees C) of inotropic and electrophysiological effects of equieffective (EC30 at 37 degrees C) concentrations of nifedipine, verapamil and cinnarizine was assessed in potassium depolarized isoprenaline-reactivated guinea-pig ventricular strips. 2. Lowering temperature greatly enhanced nifedipine inhibition of (a) maximal rate of depolarization (Vmax) of slow action potentials and (b) amplitude of contractions. 3. Electrophysiological and inotropic actions of verapamil was virtually unaffected by temperature changes. 4. Negative inotropic action of cinnarizine was greater at 37 degrees C than at lower temperature. At 37 degrees C, but not at 32 degrees C, cinnarizine reduced Vmax of slow action potentials.

On the relationship between V max of slow responses and Ca-current availability in whole-cell clamped guinea pig heart cells

The relationship between Ca current availability and maximum rate of rise (V max) of slow responses was determined in the same single guinea pig ventricular heart cell under voltage and current clamp conditions (whole-cell clamp technique). The results are as follows. (1) Cell capacitance measured in 32 cells from the current response to a fast ramp voltage-clamp pulse (119.6 +/- 4.6 pF, mean +/- SE) or from Vmax values at a holding potential of -50 or -40 mV (118.6 +/- 5.3 pF) are identical. (2) In control conditions ([Ca]o 1.8, [K]o 4 and [Cs]i 140 mM), voltage-dependence of steady-state inactivation of Ca current (ICa) or Vmax are similar up to -35 mV. However, Vmax significantly (P less than 0.005) underestimates ICa availability at more positive potentials. At -30 mV, ICa and Vmax amplitudes represent respectively 35.6 and 22.4% (n = 14) of their maximum value. (3) In the presence of 50 nM isoprenaline, Vmax and the underlying ICa are respectively increased by 79.2 +/- 13.8% and 71.2 +/- 13.8% (n = 15). No statistically significant deviation from linearity is then observed. (4) When Vmax amplitude is expressed as a function of ICa density, an almost linear relationship is observed for Vmax values between 0 and 25 V/s. Vmax is then best described by the equation: Vmax (V/s) = 1.043 ICa (pA/pF) -0.514 (46 cells). (5) We conclude that, under conditions that minimize outward currents, Vmax of slow responses accurately measures ICa amplitude, except when ICa is decreased to less than 40% of its maximum control amplitude (i.e., below 4 pA/pF). At that point, Vmax underestimates ICa.