Involvement of the IP₃ cascade in the damage to guinea-pig ventricular myocytes induced by cytotoxic T lymphocytes

Abstract

 We have shown previously that the interaction between cytotoxic T lymphocytes (CTL) and ventricular myocytes, an in vitro model for heart transplant rejection, results in electrophysiological and morphological alterations indicative of overload of the intracellular [Ca²⁺] ([Ca²⁺]_i). Since these deleterious effects cannot be accounted for by increased L-type Ca²⁺ current (I _Ca,L), we hypothesize that [Ca²⁺]_i overload due to Ca²⁺ release from intracellular stores, e.g. sarcoplasmic reticulum (SR), is initiated by CTL-induced activation of the inositol trisphosphate (IP₃) cascade. Patch-clamp and fura-2-fluorescence techniques were utilized to record transmembrane potentials and [Ca²⁺]_i from ventricular myocytes bound to peritoneal exudate CTL (PEL). In ventricular myocyte-PEL conjugates (after 60 min), resting potential was reduced (compared with the nonconjugated state) from –80.9 ± 0.7 to –59.9 ± 2.5 mV, action potential amplitude from 139.5 ± 1.4 to 80.6 ± 1.7 mV and action potential duration to 50% repolarization (APD₅₀) from 797 ± 97 to 52 ± 12 ms. The ratio of fluorescence at 340 and 380 nm (R _340/380) increased from a control value (in nonconjugated myocytes) of 0.71 ± 0.02 to 2.07 ± 0.03, 30 min after conjugate formation, and exceeded 4.0 at 60 min, before myocyte destruction. Heparin (50 μg/ml), an antagonist of IP₃-induced Ca²⁺ release from SR channels, or U-73122 (2 μM), a phospholipase C (PLC) inhibitor (drugs were included in the pipette solution), prevented PEL-induced morphological and electrophysiological alterations. Accordingly, heparin attenuated the PEL-induced increase in [Ca²⁺]_i; after 60 min of PEL-myocyte interaction, R _340/380 was 1.15 ± 0.09 (compared with approximately 4.0 in the absence of heparin). The results indicate that CTL-mediated damage to ventricular myocytes is, at least partially, mediated by PLC activation and IP₃-induced Ca²⁺ release from intracellular stores. Pharmacological targeting of IP₃ in heart transplant rejection is thus suggested.