Acclimation to hypercarbia protects cardiac contractility and alters tissue carbohydrate metabolism in the Amazonian armored catfish Pterygoplichthys pardalis

Abstract

The armored catfish Pterygoplichthys pardalis tolerates environmental hypercarbia, high partial pressures of CO₂ ($P_{{{\text{CO}}_{ 2} }}$), by preferentially protecting intracellular pH (pHi) in the face of extracellular acidosis. This response is associated with ionic changes which may disrupt contractility in cardiac muscle, and it is not known whether acclimation to hypercarbia provides protection against these changes. We studied the influence of different $P_{{{\text{CO}}_{ 2} }}$ acclimation histories on cardiac muscle function using isometrically contracting ventricular strip preparations. Fish were held for >4 months at 21 mmHg $P_{{{\text{CO}}_{ 2} }}$ and then exposed to normocarbia (6 mmHg $P_{{{\text{CO}}_{ 2} }}$) for either 15 h or 5–6 days. Acclimation to chronic hypercarbia eliminated the negative inotropic effects of in vitro hypercarbia, decreased extracellular Ca²⁺ sensitivity, and reduced maximum pacing frequency in ventricular strip preparations. Fish acclimated to chronic hypercarbia also exhibited hepatic glycogen and plasma glucose accumulation, and lower plasma lactate levels compared to fish acclimated to normocarbia for 5–6 days. We suggest chronic hypercarbia may induce cardiac remodeling to protect contractility and reduce the energetic demands of pHi regulation. The activation of HCO₃ ⁻ synthesis pathways may decrease glucose utilization and enhance carbohydrate stores, potentially providing protection against hypoxia, a stressor frequently encountered in conjunction with hypercarbia in the Amazon.