Effect of hyperosmolarity on resting and developed tension in heart muscle

CPA toxicity screening of cryoprotective solutions in rat hearts

In clinical practice, donor hearts are transported on ice prior to transplant and discarded if cold ischemia time exceeds ∼5 h. Methods to extend these preservation times are critically needed, and ideally, this storage time would extend indefinitely, enabling improved donor-to-patient matching, organ utilization, and immune tolerance induction protocols. Previously, we demonstrated successful vitrification and rewarming of whole rat hearts without ice formation by perfusion-loading a cryoprotective agent (CPA) solution prior to vitrification. However, these hearts did not recover any beating even in controls with CPA loading/unloading alone, which points to the chemical toxicity of the cryoprotective solution (VS55 in Euro-Collins carrier solution) as the likely culprit. To address this, we compared the toxicity of another established CPA cocktail (VEG) to VS55 using ex situ rat heart perfusion. The CPA exposure time was 150 min, and the normothermic assessment time was 60 min. Using Celsior as the carrier, we observed partial recovery of function (atria-only beating) for both VS55 and VEG. Upon further analysis, we found that the VEG CPA cocktail resulted in 50 % lower LDH release than VS55 (N = 4, p = 0.017), suggesting VEG has lower toxicity than VS55. Celsior was a better carrier solution than alternatives such as UW, as CPA + Celsior-treated hearts spent less time in cardiac arrest (N = 4, p = 0.029). While we showed substantial improvement in cardiac function after exposure to vitrifiable concentrations of CPA by improving both the CPA and carrier solution formulation, further improvements will be required before we achieve healthy cryopreserved organs for transplant.

Coronary flow-induced inotropism is modulated by binding of dextrans to the endothelial luminal surface

In isolated perfused guinea pig hearts, coronary flow causes a positive inotropic effect [positive coronary flow-induced effect (+CFIE)] that could be altered by dextrans (Dx) in the coronary perfusion solution. To test this possibility, Dx of 20, 40, 70, and 500 kDa were infused and found to modulate +CFIE; however, when Dx infusion was terminated, the effect persisted, i.e., was irreversible/nonwashable, suggesting that Dx may bind to luminal endothelial lectinic structures. This hypothesis was tested when Dx [with fluorescent traces (D*)] bound to the vessel wall was hydrolyzed by dextranase infusion and washout of D* fragments completely reverted the +CFIE, and it was found that bound D* to be displaced by free Dx required concentrations 50-100 times that used during binding. In addition, dose-response curves for Dx on +CFIE show that the higher the Dx molecular mass, the lesser the concentration required to have an effect. Because a large Dx molecule has a greater number polymeric glucose branches, it can bind to a larger number of endothelial lectinic sites, requiring a lower concentration to affect +CFIE. Our results suggest that luminal endothelial lectinic structures are part of the flow-sensing assembly.

Restoration of osmotically inhibited twitch force in rat cardiac trabeculae: role of Na+-H+ exchange

1. When rat cardiac muscle is subjected to an increase of osmolality, its peak twitch force is immediately inhibited. Subsequently, over a period of several minutes, twitch force undergoes restoration, the extent of which is determined by the osmolality. The aim of the present study was to determine the factors that contribute to this restorative phenomenon. 2. Trabeculae were isolated from the right ventricles of rat hearts and mounted in an organ bath at 37 degrees C. The osmolality of the bathing solution was increased by 100 mOsmol (to 400 mOsmol) by the addition of various proportions of NaCl and sucrose while recording twitch force production. The role of Na+-H+ exchange in restoring twitch force was examined by use of the specific inhibitor cariporide (HOE 642). The role of Na+-Ca2+ exchange was examined by reducing [Ca2+]o (from 2 mmol/L to 0.5 mmol/L) or by substituting LiCl for NaCl. 3. Cariporide (25 micro mol/L) completely abolished twitch force restoration, thereby implicating a central role for the Na+-H+ exchanger. At constant [Na+]o, the extent of restoration was [Ca2+]o dependent, suggesting an independent contribution by the Na+-Ca2+ exchanger. This suggestion was supported by the finding that Li+, which substitutes for Na+ on the Na+-H+ exchanger, but not on the Na+-Ca2+ exchanger, also reduced the extent of restoration of hyperosmotically inhibited twitch force. 4. We conclude that the immediate inhibition of peak twitch force of rat cardiac muscle by hyperosmotic solutions reflects, in part, elevation of [H+]i, subsequent to reduction of cell volume. Hyperosmotic activation of Na+-H+ exchange then progressively relieves the inhibitory effect of protons on force development. The accompanying increase in [Na+]i in turn enhances Ca2+ influx on the Na+-Ca2+ exchanger, with the result that twitch force undergoes further restoration.