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Thrombin modulates phosphoinositide metabolism, cytosolic calcium, and impulse initiation in the heart
S F Steinberg1, R B Robinson, H B Lieberman
1Department of Medicine, Center for Radiological Research, Columbia University, New York, NY 10032.
Insights
Thrombin affects heart cells by increasing calcium and altering electrical activity, potentially contributing to heart problems during ischemia. This study reveals new roles for thrombin in cardiac function.
Area of Science:
- Cardiovascular Biology
- Cellular Physiology
- Biochemistry
Background:
- Thrombin is known to stimulate cellular processes like phosphoinositide hydrolysis and calcium release in various cell types.
- The specific effects of thrombin on cardiac cells and its link to cardiac electrical activity remain largely unexplored.
Purpose of the Study:
- To investigate the biochemical and electrophysiological effects of thrombin on the mammalian heart.
- To determine if thrombin modulates cardiac phosphoinositide hydrolysis, calcium metabolism, and electrical activity.
Main Methods:
- Utilized neonatal rat ventricular myocyte cultures and canine Purkinje fibers.
- Employed microelectrode techniques and the fluorescent calcium indicator fura-2.
- Examined the impact of thrombin on phosphoinositide breakdown, action potential duration, and intracellular calcium levels.
Main Results:
- Thrombin rapidly induced phosphoinositide hydrolysis in myocytes, independent of cAMP levels.
- In Purkinje fibers, thrombin increased beating rate in depolarized cells, prolonged action potential duration, and augmented afterdepolarizations.
- Thrombin elevated both diastolic and systolic intracellular calcium in various cardiac cell types.
Conclusions:
- Thrombin exerts novel stimulatory actions on the mammalian heart, affecting both biochemical pathways and electrophysiological properties.
- Thrombin's modulation of cardiac calcium and electrical activity, including enhanced automaticity and prolonged repolarization, may play a role in electrical abnormalities during myocardial ischemia and infarction.
Abstract:
Thrombin stimulates phosphoinositide hydrolysis and increases cytosolic calcium in several types of cells. To determine whether thrombin exerts similar stimulatory actions in the heart and whether this mechanism is linked to changes in cardiac electrical activity, the effects of thrombin on several biochemical and electrophysiological parameters were examined. In neonatal rat ventricular myocyte cultures freed of fibroblast contamination by irradiation, thrombin rapidly induced the breakdown of phosphoinositides. Formation of inositol trisphosphate was detectable within 5 seconds and was followed by the sequential accumulation of inositol bisphosphate and inositol monophosphate. The effect of thrombin to stimulate phosphoinositide hydrolysis was inhibited by hirudin, but not by propranolol, prazosin, or pretreatment with pertussis toxin. The inositol phospholipid response was unassociated with changes in intracellular cAMP levels. To determine the electrophysiological effects of thrombin, we used microelectrode techniques to study canine Purkinje fibers. Thrombin increased the beating rate of fibers depolarized using barium, but not those at normal maximal diastolic potential. In addition, thrombin prolonged the action potential duration in fibers driven at a constant cycle length. This response was inhibited by hirudin and nisoldipine, but not by propranolol, prazosin, or pretreatment with pertussis toxin. Thrombin also augmented cesium-induced early afterdepolarizations. Using the fluorescent calcium indicator fura-2, we demonstrated that thrombin increased the beating rate, diastolic calcium, and peak systolic calcium of spontaneously contracting cultured ventricular myocytes. Cytosolic calcium also increased in both rat ventricular myocytes and canine Purkinje myocytes that were electrically driven at a constant basic cycle length, indicating that thrombin modulates cellular calcium metabolism independent of its actions to enhance automaticity. Taken together, these findings demonstrate several novel biological actions of thrombin in the mammalian heart that may be functionally related. The actions of thrombin to enhance automaticity and prolong repolarization may contribute to the electrical abnormalities observed in the setting of myocardial ischemia and infarction.