Changes in cardiac mechanics with heat acclimation: Adrenergic signaling and SR-Ca regulatory proteins

The involvement of adrenergic signaling and sarcoplasmic calcium regulatory proteins in the development of heat acclimation-induced adaptations in cardiac mechanics was studied in heat-acclimated (34°C) rats for 2, 5, and 30 days (AC₂, AC₅ and AC₃₀, respectively). Control (C) rats were held at 24 ± 1°C. Systolic pressure (LVP) and velocities of contraction (dP/dt/P) and relaxation (-dP/dt/P) were measured using a Langendorff system. For adrenergic signaling, β-adrenoreceptor (AR) density and affinity (Scatchard plots) and cardiac inotropic response to norepinephrine (10^-7 mM, ± 10^-6 mM propranolol) were measured. For the regulatory proteins, steady-state levels of Ca²⁺-ATPase and phospholamban (PLB) mRNAs and the encoded proteins Ca²⁺-ATPase [sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA)] and PLB were measured using semiquantitative RT-PCR and Western immunoblotting, respectively. Both short (STHA; AC₂ and AC₅)- and long-term heat acclimation (LTHA; AC₃₀) enhanced LVP. However, dP/dt·P and -dP/dt·P in STHA hearts resembled that of the controls, whereas on LTHA, both parameters decreased (P < 0.05), implying decreased velocity of contraction and relaxation. β-AR density remained unchanged with their affinity markedly decreased (P < 0.05). AR responsiveness, however, diminished in AC₂ but was markedly enhanced on LTHA. During STHA, PLB and sarcoplasmic reticulum Ca²⁺-ATPase transcripts were upregulated with no change in the encoded proteins except for SERCA downregulation on AC5, leading to an increased PLB/SERCA ratio (P < 0.05). This mismatched preacclimation lusitropic state on STHA and increased PLB/SERCA ratio was evident (P < 0.05) due to downregulation of SERCA and upregulation of PLB. Our data fit a biphasic acclimation model in which desensitized adrenergic signaling is dominant during STHA, whereas on LTHA, the contractile machinery is influenced by altered expression of the calcium regulatory proteins leading to both augmented adrenergic inotropic response (via PLB elevation) and decreased velocity of relaxation. The sustained low thyroxin measured on LTHA causally associates with this response.