Research Interests

(under revision)

The research program in the Cardiac Membrane Research Lab (CMRL) focuses on the cellular and molecular mechanisms by which the heart adapts to physiological, pathological and environmental perturbations. The strength of cardiac muscle contraction is regulated by the cytosolic Ca²⁺ activity ([Ca²⁺]_i) on a beat-to-beat basis. We focus, therefore, on factors which control [Ca²⁺]_i in the heart. These studies are multidisciplinary and conducted at three different levels of organization. The first and most integrative approach makes use of isolated single cardiac cells in which speed of shortening is assessed by video microscopy using edge detection and the [Ca²⁺]_i transient is determined on line with indo-1 fluorescence microscopy. The second approach involves measuring Ca²⁺ movement across isolated membranes using voltage clamp, planar lipid bilayers and/or radioisotopic techniques. The third approach includes the cloning, sequencing and expression of proteins critically involved in Ca²⁺ handling. These proteins include the: L-type voltage-dependent Ca²⁺ channel (DHPR), which controls Ca²⁺ influx; Na⁺/Ca²⁺ exchanger (NCX), which is the primary mechanism of Ca²⁺ efflux; and troponin C, a Ca²⁺ binding protein which is crucial in the initiation of contraction. These proteins and mutants produced by site-directed mutagenesis are studied in vitro as well as in the reconstituted muscle fibre. With these techniques, we are investigating the mechanisms by which these proteins are regulated in different species to meet a variety of physiological, pathological and environmental demands.

ONTOGENY OF CARDIAC CONTRACTILITY REGULATION

The mortality associated with open heart surgery is 2-3 fold higher in neonates than in adults. This is due in part to the dearth of knowledge about neonate cardiac function and its response to the ischemia reperfusion insult that occurs by necessity in open heart surgery. The neonate heart is more susceptible to damage following ischemia and reperfusion because of the inability of the cardiomyocytes to control cytosolic H⁺ (pH_i) and [Ca²⁺]_i levels. For example, in the early stages of ontogeny, the SR is poorly developed as is the functional interaction between the The elevated [Ca²⁺]_i is due, in part, intracellular acidosis which results in elevated intracellular Na⁺ levels ([Na⁺]_i) through the stimulated activity of the Na⁺/H⁺ exchanger. The high [Na⁺]_i in turn supports reverse mode of Na⁺/Ca²⁺ exchange raising [Ca²⁺]_i. The predominance of the Na⁺/Ca²⁺ exchanger and the Na⁺/H⁺ antiporter in the neonate may lend itself to strategies aimed at blocking the effect of these systems upon sodium influx and subsequent calcium influx during ischemia and reperfusion. Elevated [Ca²⁺]_i initiates a variety of destructive events which can lead to death in the cell. The neonate may gain from these strategies to a greater extent than the adult.

PHYLOGENY OF CARDIAC CONTRACTILITY REGULATION

We have developed an interest in the evolution of the regulatory mechanisms of [Ca²⁺]_i the heart. In these studies, we primarily use rainbow trout and tuna (which precede humans by ~400 million years) as a model. These species allow for unique physiological and culinary experiments. Fish, being poikilotherms, are exposed to and must function under a wide range of environmental temperatures and pH. For example, the hearts of active teleosts such as the salmonids contract robustly under hypothermic conditions which are cardioplegic to mammals. The mechanisms of the adaptive responses in these species to the diverse physiological conditions are largely unknown and are a focus of our research efforts. In addition, the parallels in myocyte structure and function between mammalian neonates and adult lower vertebrates are striking and conform to the embryologist's old adage that ontogeny recapitulates phylogeny.