Calcium flux in turtle ventricular myocytes

Area of Science:

• Cardiovascular Physiology
• Cellular Electrophysiology
• Comparative Biology

Background:

• Understanding calcium handling is crucial for cardiac function.
• The relative roles of sarcoplasmic reticulum (SR), L-type Ca(2+) channels, and Na(+)/Ca(2+) exchanger (NCX) in cardiac contraction vary across species.
• Turtle ventricular myocytes possess unique structural and functional characteristics influencing calcium dynamics.

Purpose of the Study:

• To determine the contribution of SR, L-type Ca(2+) channels, and NCX to contraction and relaxation in turtle ventricular myocytes.
• To investigate the mechanisms of calcium influx and handling in a non-mammalian vertebrate heart.

Main Methods:

• Epifluorescent microscopy with Fura-2 dye to visualize Ca(2+) transients.
• Confocal microscopy to analyze myocyte structure (e.g., T-tubules).
• Perforated-patch voltage-clamp electrophysiology to measure sarcolemmal Ca(2+) currents (I(Ca)) and NCX activity.

Main Results:

• Turtle ventricular myocytes lack T-tubules and show Ca(2+) transients insensitive to SR inhibitors (ryanodine, thapsigargin), indicating a minimal SR role.
• Significant L-type Ca(2+) channel current (I(Ca)) density (3.2 ± 0.5 pA/pF) contributes substantially to Ca(2+) influx.
• Na(+)/Ca(2+) exchanger (NCX) activity is significant, with Ca(2+) influx varying with intracellular Na(+) concentrations (58.5 ± 7.7 µmol/l at 14 mM Na(+) vs. 26.7 ± 3.2 µmol/l at 7 mM Na(+)).

Conclusions:

• Turtle ventricular myocytes are adapted for substantial sarcolemmal Ca(2+) transport.
• The L-type Ca(2+) channel is the primary source of Ca(2+) for contraction in turtle ventricular myocytes.
• The NCX can independently support myocyte contraction, especially at higher intracellular Na(+) levels, in the absence of SR and L-type Ca(2+) channel contributions.