Calcium binding to cardiac myocytes protected from proteolytic enzyme activity

Excitation-contraction coupling in cardiac muscle depends on extracellular calcium and calcium bound to the surface of myocardial cells. Prior research has shown that calcium binding to sarcolemmal structures is essential for initiating contraction. However, the specific pools of calcium binding and their characteristics remain unclear. Established methods have identified surface calcium pools but lack specificity in distinguishing their functional roles. This gap motivated researchers to investigate calcium binding pools in isolated cardiac myocytes. The study aimed to clarify whether these pools are affected by proteolytic enzymes or surface modifications. No prior work had resolved how enzyme treatments influence calcium binding affinity and capacity. The current research builds on existing knowledge by exploring the structural basis of calcium binding in cardiac cells.

The study sought to examine calcium binding characteristics in adult guinea pig ventricular myocytes. Researchers aimed to identify distinct calcium binding pools and their properties. They focused on whether these pools are affected by proteolytic enzyme activity. The motivation stemmed from a need to clarify the role of surface-bound calcium in excitation-contraction coupling. The research also aimed to determine if enzyme treatments or surface modifications alter binding affinity. By isolating myocytes and using equilibrium dialysis, the team could measure binding parameters. This approach allowed them to distinguish high- and low-affinity pools. The ultimate goal was to understand how these pools contribute to cardiac function.

The researchers used mechanically disaggregated adult guinea pig ventricular myocytes. Cells were isolated in an oxygenated tissue culture medium containing aprotinin, a proteinase inhibitor. Cytodex beads were used to separate myocytes from cellular debris. Calcium binding was measured using continuous flow equilibrium dialysis. Scatchard plots were generated to determine binding affinity and capacity. The study tested the effects of lanthanum, trypsin, and collagenase on calcium binding. Neuraminidase and phospholipase C were also applied to assess their impact. The team analyzed how these treatments altered binding parameters in isolated myocytes.

Scatchard plots revealed two distinct calcium binding pools in the myocytes. The high-affinity pool had a Ka of 65 X 10(3) M-1 and a Bt of 1.3 nmol X mg-1. The low-affinity pool had a Ka of 141 M-1 and a Bt of 138 nmol X mg-1. Lanthanum, trypsin, and collagenase eliminated the low-affinity pool. Aprotinin in the isolation medium was necessary to detect this pool. Neuraminidase and phospholipase C reduced the Bt of the low-affinity pool by half. Neuraminidase also increased the Ka of this pool to 516.7 M-1. This value matched the apparent affinity constant estimated from dP/dtmax measurements at various calcium concentrations.

The findings suggest that calcium bound to sarcolemmal phospholipids represents the superficial calcium involved in excitation-contraction coupling. The low-affinity pool was sensitive to proteolytic enzyme activity and surface modifications. The high-affinity pool remained unaffected by these treatments. The study supports the idea that surface-bound calcium plays a role in initiating cardiac contractions. The results align with prior research on calcium signaling in cardiac cells. The presence of aprotinin in the isolation medium was crucial for detecting the low-affinity pool. Neuraminidase altered both the affinity and capacity of this pool. These findings contribute to understanding how calcium binding influences heart function.