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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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Stimulus waveform (A) and representative Na+/Ca2+ exchanger current (INCX) (B) from a lizard ventricular myocyte (40 pF) elicited by square depolarizing pulses to 0 and 20 mV (with a prepulse to −40 mV to remove INa). B: calculated total Ca2+ transferred by INCX during a pulse to 0 or 20 mV expressed as μmol/l nonmyofibrillar (see methods). *Significantly more Ca2+ transferred on INCX at 20 mV compared with 0 mV. Values are means ± SE, n = 7.
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Figure 6: Stimulus waveform (A) and representative Na+/Ca2+ exchanger current (INCX) (B) from a lizard ventricular myocyte (40 pF) elicited by square depolarizing pulses to 0 and 20 mV (with a prepulse to −40 mV to remove INa). B: calculated total Ca2+ transferred by INCX during a pulse to 0 or 20 mV expressed as μmol/l nonmyofibrillar (see methods). *Significantly more Ca2+ transferred on INCX at 20 mV compared with 0 mV. Values are means ± SE, n = 7.

Mentions: A square-wave voltage pulse from −40 mV to 0 mV for 500 ms and then to +20 mV for 500 ms (Fig. 6A) gave rise to a maintained outward current, which could be blocked with 10 mM NiCl2, confirming the presence of a Ni+-sensitive Na/Ca2+ exchange current (INCX) (Fig. 6B). INCX was integrated to give a measure of charge transfer so that total Ca2+ influx through the NCX could be calculated. Charge transfer was significantly greater at 20 mV (Fig. 6C).


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)

Stimulus waveform (A) and representative Na+/Ca2+ exchanger current (INCX) (B) from a lizard ventricular myocyte (40 pF) elicited by square depolarizing pulses to 0 and 20 mV (with a prepulse to −40 mV to remove INa). B: calculated total Ca2+ transferred by INCX during a pulse to 0 or 20 mV expressed as μmol/l nonmyofibrillar (see methods). *Significantly more Ca2+ transferred on INCX at 20 mV compared with 0 mV. Values are means ± SE, n = 7.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 6: Stimulus waveform (A) and representative Na+/Ca2+ exchanger current (INCX) (B) from a lizard ventricular myocyte (40 pF) elicited by square depolarizing pulses to 0 and 20 mV (with a prepulse to −40 mV to remove INa). B: calculated total Ca2+ transferred by INCX during a pulse to 0 or 20 mV expressed as μmol/l nonmyofibrillar (see methods). *Significantly more Ca2+ transferred on INCX at 20 mV compared with 0 mV. Values are means ± SE, n = 7.
Mentions: A square-wave voltage pulse from −40 mV to 0 mV for 500 ms and then to +20 mV for 500 ms (Fig. 6A) gave rise to a maintained outward current, which could be blocked with 10 mM NiCl2, confirming the presence of a Ni+-sensitive Na/Ca2+ exchange current (INCX) (Fig. 6B). INCX was integrated to give a measure of charge transfer so that total Ca2+ influx through the NCX could be calculated. Charge transfer was significantly greater at 20 mV (Fig. 6C).

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