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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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A: current-voltage relationship for INCX in lizard ventricular cardiomyocytes. The current is calculated from the voltage ramp pulse (see inset). •, Ni-sensitive current (10 mM NiCl2); ○, KB-R7943 (KBR)-sensitive current (KBR, 5 μM). *Significant block by KBR (2-way repeated-measures ANOVA, P < 0.05). B: %inhibition of forward and reverse INCX by 5 μM KBR calculated from the KBR-sensitive current as a fraction of INCX at 50 mV on either side of the reversal potential of −10 mV. *Significantly greater inhibition of reverse INCX. All values are means ± SE, n = 8–10 cells.
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Figure 7: A: current-voltage relationship for INCX in lizard ventricular cardiomyocytes. The current is calculated from the voltage ramp pulse (see inset). •, Ni-sensitive current (10 mM NiCl2); ○, KB-R7943 (KBR)-sensitive current (KBR, 5 μM). *Significant block by KBR (2-way repeated-measures ANOVA, P < 0.05). B: %inhibition of forward and reverse INCX by 5 μM KBR calculated from the KBR-sensitive current as a fraction of INCX at 50 mV on either side of the reversal potential of −10 mV. *Significantly greater inhibition of reverse INCX. All values are means ± SE, n = 8–10 cells.

Mentions: The voltage-dependence of INCX was measured using a voltage ramp protocol (Fig. 7A, inset). INCX was identified as the Ni+-sensitive current. INCX showed a steep increase with increasing voltage (outward rectification) (Fig. 7A), while the inward current increased more slowly during hyperpolariazation. The measured reversal potential of INCX was −10 mV. Using this reversal potential, we calculate an intracellular-free Ca2+ of ∼50 nM. This value is lower than the diastolic value obtained in Fura-2-loaded cells (∼150 nM) and may be due to the fact cells were not undergoing constant stimulation. Certainly a higher intracellular-free Δ[Ca2+] would have shifted the reversal potential of the NCX to more positive voltages.


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)

A: current-voltage relationship for INCX in lizard ventricular cardiomyocytes. The current is calculated from the voltage ramp pulse (see inset). •, Ni-sensitive current (10 mM NiCl2); ○, KB-R7943 (KBR)-sensitive current (KBR, 5 μM). *Significant block by KBR (2-way repeated-measures ANOVA, P < 0.05). B: %inhibition of forward and reverse INCX by 5 μM KBR calculated from the KBR-sensitive current as a fraction of INCX at 50 mV on either side of the reversal potential of −10 mV. *Significantly greater inhibition of reverse INCX. All values are means ± SE, n = 8–10 cells.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 7: A: current-voltage relationship for INCX in lizard ventricular cardiomyocytes. The current is calculated from the voltage ramp pulse (see inset). •, Ni-sensitive current (10 mM NiCl2); ○, KB-R7943 (KBR)-sensitive current (KBR, 5 μM). *Significant block by KBR (2-way repeated-measures ANOVA, P < 0.05). B: %inhibition of forward and reverse INCX by 5 μM KBR calculated from the KBR-sensitive current as a fraction of INCX at 50 mV on either side of the reversal potential of −10 mV. *Significantly greater inhibition of reverse INCX. All values are means ± SE, n = 8–10 cells.
Mentions: The voltage-dependence of INCX was measured using a voltage ramp protocol (Fig. 7A, inset). INCX was identified as the Ni+-sensitive current. INCX showed a steep increase with increasing voltage (outward rectification) (Fig. 7A), while the inward current increased more slowly during hyperpolariazation. The measured reversal potential of INCX was −10 mV. Using this reversal potential, we calculate an intracellular-free Ca2+ of ∼50 nM. This value is lower than the diastolic value obtained in Fura-2-loaded cells (∼150 nM) and may be due to the fact cells were not undergoing constant stimulation. Certainly a higher intracellular-free Δ[Ca2+] would have shifted the reversal potential of the NCX to more positive voltages.

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