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Characteristics of Prolonged Ca²⁺ Release Events Associated With the Nuclei in Adult Cardiac Myocytes

From the School of Biomedical Sciences, University of Leeds, Leeds, UK.

Correspondence to Derek Steele, School of Biomedical Sciences, University of Leeds, Leeds LS2 9JT, UK. E-mail d.steele{at}leeds.ac.uk

Confocal microscopy was used to study the properties of nuclear Ca²⁺ regulation in adult ventricular myocytes. Prolonged nuclear Ca²⁺ release (PNCR) events were identified in both intact and permeabilized rat myocytes. PNCR occurred spontaneously and was restricted to localized regions at the ends of the elongated nuclei. Typically, PNCR took the form of a rapid rise in [Ca²⁺] followed by a maintained plateau. The mean duration of PNCR (1.78±0.19 seconds) was markedly greater than the half decay time for cytosolic Ca²⁺ sparks (31.2±0.56 ms) obtained under the same conditions. The PNCR width at half maximum amplitude (5.0±0.2 µm) was also significantly greater than that of cytosolic Ca²⁺ sparks (2.6±0.05 µm) obtained under the same conditions. Experiments involving the use of syto-11 to accurately locate the nuclei demonstrated that PNCR originates from the nuclear envelope or a closely associated structure. The spatial spread of PNCR was asymmetrical, with greater diffusion of Ca²⁺ toward the center of the nucleus than the cytosol. Both PNCR and Ca²⁺ sparks were abolished by interventions that deplete SR Ca²⁺ stores or inhibit RYR activation. Experiments on intact, electrically stimulated cells revealed that diffusion of Ca²⁺ from the ends of the nucleus toward the center is a prominent feature of the nucleoplasmic Ca²⁺ transient. The possibility that recruitment of Ca²⁺ release sites involved in PNCR might influence the temporal and spatial characteristics of the nucleoplasmic [Ca²⁺] transient is considered.

Key Words: nucleus • sarcoplasmic reticulum • Ca²⁺ release • sparks

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