IP₃R activity increases propensity of RyR-mediated sparks by elevating dyadic [Ca^2＋]

Area of Science:

• Cardiovascular Physiology
• Molecular Cardiology
• Computational Biology

Background:

• Calcium (Ca2+) is crucial for cardiomyocyte contraction via excitation-contraction coupling (ECC).
• Ryanodine receptors (RyRs) mediate primary Ca2+ release, but inositol 1,4,5-trisphosphate receptors (IP3Rs) also modulate ECC in cardiomyocytes.
• Dyadic localization of IP3Rs suggests a role in regulating Ca2+ sparks, the elementary events of ECC.

Purpose of the Study:

• To elucidate the mechanism by which dyad-localized IP3Rs influence Ca2+ spark dynamics.
• To understand how IP3R activity impacts local Ca2+ handling and Ca2+ spark properties within cardiomyocyte dyads.

Main Methods:

• Development of a mathematical model of the cardiac dyad.
• Incorporation of both IP3R and RyR behavior into the model.
• Simulation of local Ca2+ handling, Ca2+ spark occurrence, and spark properties.

Main Results:

• Model predicts increased Ca2+ spark formation propensity with IP3R activity, consistent with experimental data.
• IP3Rs sensitize proximal RyRs to activation by elevating local dyadic Ca2+.
• IP3R activity leads to Ca2+ sparks with similar durations but reduced amplitudes due to junctional sarcoplasmic reticulum Ca2+ depletion.

Conclusions:

• Dyadic IP3Rs modulate Ca2+ spark dynamics by influencing RyR activation and Ca2+ availability.
• The stochastic gating of IP3Rs is a key factor in their modulatory mechanism.
• IP3R activity fine-tunes Ca2+ release events, impacting cardiomyocyte excitation-contraction coupling.