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Ca2+ cycling in cardiomyocytes from a high-performance reptile, the varanid lizard (Varanus exanthematicus).

Galli GL, Warren DE, Shiels HA - Am. J. Physiol. Regul. Integr. Comp. Physiol. (2009)

Bottom Line: Specializations in excitation-contraction coupling may also contribute to the varanids superior cardiovascular performance.Lizard ventricular myocytes were found to be spindle-shaped, lack T-tubules, and were approximately 190 microm in length and 5-7 microm in width and depth.In aggregate, our results suggest varanids have an enhanced capacity to transport Ca(2+) through the Na(+)/Ca(2+) exchanger, and sarcoplasmic reticulum suggesting specializations in excitation-contraction coupling may provide a means to support high cardiovascular performance.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Life Sciences, The University of Manchester, Core Technology Facility, Manchester, United Kingdom. ggalli@interchange.ubc.ca

ABSTRACT
The varanid lizard possesses one of the largest aerobic capacities among reptiles with maximum rates of oxygen consumption that are twice that of other lizards of comparable sizes at the same temperature. To support this aerobic capacity, the varanid heart possesses morphological adaptations that allow the generation of high heart rates and blood pressures. Specializations in excitation-contraction coupling may also contribute to the varanids superior cardiovascular performance. Therefore, we investigated the electrophysiological properties of the l-type Ca(2+) channel and the Na(+)/Ca(2+) exchanger (NCX) and the contribution of the sarcoplasmic reticulum to the intracellular Ca(2+) transient (Delta[Ca(2+)](i)) in varanid lizard ventricular myocytes. Additionally, we used confocal microscopy to visualize myocytes and make morphological measurements. Lizard ventricular myocytes were found to be spindle-shaped, lack T-tubules, and were approximately 190 microm in length and 5-7 microm in width and depth. Cardiomyocytes had a small cell volume ( approximately 2 pL), leading to a large surface area-to-volume ratio (18.5), typical of ectothermic vertebrates. The voltage sensitivity of the l-type Ca(2+) channel current (I(Ca)), steady-state activation and inactivation curves, and the time taken for recovery from inactivation were also similar to those measured in other reptiles and teleosts. However, transsarcolemmal Ca(2+) influx via reverse mode Na(+)/Ca(2+) exchange current was fourfold higher than most other ectotherms. Moreover, pharmacological inhibition of the sarcoplasmic reticulum led to a 40% reduction in the Delta[Ca(2+)](i) amplitude, and slowed the time course of decay. In aggregate, our results suggest varanids have an enhanced capacity to transport Ca(2+) through the Na(+)/Ca(2+) exchanger, and sarcoplasmic reticulum suggesting specializations in excitation-contraction coupling may provide a means to support high cardiovascular performance.

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Steady-state activation and inactivation of ICa in varanid lizard ventricular cardiac cardiomyocytes. A: voltage protocol used to measure steady-state activation and inactivation. B: representative current recording from a varanid ventricular myocyte (capacitance = 56.2 pF) subjected to the voltage protocol given in A. C: mean steady-state inactivation and activation profiles. Values are means ± SE (n = 12). Inactivation (▪) is measured by depolarizing from −70 mV to a test potential for 1 s and then testing the remaining available ICa at 0 mV. Activation (□) is calculated by dividing peak current by apparent driving force [applied membrane potential (Em) − reversal potential (Erev)] according to Ohm's law. Inset: l-type Ca2+ channel window current (product of activation and inactivation at each voltage).
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Figure 4: Steady-state activation and inactivation of ICa in varanid lizard ventricular cardiac cardiomyocytes. A: voltage protocol used to measure steady-state activation and inactivation. B: representative current recording from a varanid ventricular myocyte (capacitance = 56.2 pF) subjected to the voltage protocol given in A. C: mean steady-state inactivation and activation profiles. Values are means ± SE (n = 12). Inactivation (▪) is measured by depolarizing from −70 mV to a test potential for 1 s and then testing the remaining available ICa at 0 mV. Activation (□) is calculated by dividing peak current by apparent driving force [applied membrane potential (Em) − reversal potential (Erev)] according to Ohm's law. Inset: l-type Ca2+ channel window current (product of activation and inactivation at each voltage).

Mentions: The voltage protocol for measuring steady-state activation and inactivation of ICa and a representative current recording is given in Fig. 4, A and B, respectively. Activation of ICa began positive to −40 mV, and was half maximal (Vh) at −9.5 ± 1.4 mV, while inactivation of ICa, or channel availability, began decreasing positive to −30 mV and was half complete at −24.5 ± 1.0 mV (Fig. 4C). The slopes of activation and inactivation (k) were 5.6 ± 0.2 and 3.8 ± 0.1, respectively. At voltages positive to 10 mV, channel inactivation was attenuated, probably due to a reduced driving force and consequently less Ca2+-dependent inactivation (Fig. 4C). As a result of overlap between activation and inactivation curves, a window current was evident between −40 and 10 mV. The window current was maximal at approximately −18 mV, where it contributed 4% of maximal conductance (Fig. 4C, inset).


Ca2+ cycling in cardiomyocytes from a high-performance reptile, the varanid lizard (Varanus exanthematicus).

Galli GL, Warren DE, Shiels HA - Am. J. Physiol. Regul. Integr. Comp. Physiol. (2009)

Steady-state activation and inactivation of ICa in varanid lizard ventricular cardiac cardiomyocytes. A: voltage protocol used to measure steady-state activation and inactivation. B: representative current recording from a varanid ventricular myocyte (capacitance = 56.2 pF) subjected to the voltage protocol given in A. C: mean steady-state inactivation and activation profiles. Values are means ± SE (n = 12). Inactivation (▪) is measured by depolarizing from −70 mV to a test potential for 1 s and then testing the remaining available ICa at 0 mV. Activation (□) is calculated by dividing peak current by apparent driving force [applied membrane potential (Em) − reversal potential (Erev)] according to Ohm's law. Inset: l-type Ca2+ channel window current (product of activation and inactivation at each voltage).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2803631&req=5

Figure 4: Steady-state activation and inactivation of ICa in varanid lizard ventricular cardiac cardiomyocytes. A: voltage protocol used to measure steady-state activation and inactivation. B: representative current recording from a varanid ventricular myocyte (capacitance = 56.2 pF) subjected to the voltage protocol given in A. C: mean steady-state inactivation and activation profiles. Values are means ± SE (n = 12). Inactivation (▪) is measured by depolarizing from −70 mV to a test potential for 1 s and then testing the remaining available ICa at 0 mV. Activation (□) is calculated by dividing peak current by apparent driving force [applied membrane potential (Em) − reversal potential (Erev)] according to Ohm's law. Inset: l-type Ca2+ channel window current (product of activation and inactivation at each voltage).
Mentions: The voltage protocol for measuring steady-state activation and inactivation of ICa and a representative current recording is given in Fig. 4, A and B, respectively. Activation of ICa began positive to −40 mV, and was half maximal (Vh) at −9.5 ± 1.4 mV, while inactivation of ICa, or channel availability, began decreasing positive to −30 mV and was half complete at −24.5 ± 1.0 mV (Fig. 4C). The slopes of activation and inactivation (k) were 5.6 ± 0.2 and 3.8 ± 0.1, respectively. At voltages positive to 10 mV, channel inactivation was attenuated, probably due to a reduced driving force and consequently less Ca2+-dependent inactivation (Fig. 4C). As a result of overlap between activation and inactivation curves, a window current was evident between −40 and 10 mV. The window current was maximal at approximately −18 mV, where it contributed 4% of maximal conductance (Fig. 4C, inset).

Bottom Line: Specializations in excitation-contraction coupling may also contribute to the varanids superior cardiovascular performance.Lizard ventricular myocytes were found to be spindle-shaped, lack T-tubules, and were approximately 190 microm in length and 5-7 microm in width and depth.In aggregate, our results suggest varanids have an enhanced capacity to transport Ca(2+) through the Na(+)/Ca(2+) exchanger, and sarcoplasmic reticulum suggesting specializations in excitation-contraction coupling may provide a means to support high cardiovascular performance.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Life Sciences, The University of Manchester, Core Technology Facility, Manchester, United Kingdom. ggalli@interchange.ubc.ca

ABSTRACT
The varanid lizard possesses one of the largest aerobic capacities among reptiles with maximum rates of oxygen consumption that are twice that of other lizards of comparable sizes at the same temperature. To support this aerobic capacity, the varanid heart possesses morphological adaptations that allow the generation of high heart rates and blood pressures. Specializations in excitation-contraction coupling may also contribute to the varanids superior cardiovascular performance. Therefore, we investigated the electrophysiological properties of the l-type Ca(2+) channel and the Na(+)/Ca(2+) exchanger (NCX) and the contribution of the sarcoplasmic reticulum to the intracellular Ca(2+) transient (Delta[Ca(2+)](i)) in varanid lizard ventricular myocytes. Additionally, we used confocal microscopy to visualize myocytes and make morphological measurements. Lizard ventricular myocytes were found to be spindle-shaped, lack T-tubules, and were approximately 190 microm in length and 5-7 microm in width and depth. Cardiomyocytes had a small cell volume ( approximately 2 pL), leading to a large surface area-to-volume ratio (18.5), typical of ectothermic vertebrates. The voltage sensitivity of the l-type Ca(2+) channel current (I(Ca)), steady-state activation and inactivation curves, and the time taken for recovery from inactivation were also similar to those measured in other reptiles and teleosts. However, transsarcolemmal Ca(2+) influx via reverse mode Na(+)/Ca(2+) exchange current was fourfold higher than most other ectotherms. Moreover, pharmacological inhibition of the sarcoplasmic reticulum led to a 40% reduction in the Delta[Ca(2+)](i) amplitude, and slowed the time course of decay. In aggregate, our results suggest varanids have an enhanced capacity to transport Ca(2+) through the Na(+)/Ca(2+) exchanger, and sarcoplasmic reticulum suggesting specializations in excitation-contraction coupling may provide a means to support high cardiovascular performance.

Show MeSH
Related in: MedlinePlus