Mice release less tissue-type plasminogen activator (t-PA) and have more active clot stabilization by FXIII compared to rats which might explain their low rate of blood clot lysis

Abstract

Fibrinolysis regulates the removal of blood clots from the bloodstream and is mediated by tissue-type plasminogen activator (t-PA) and its inhibitor plasminogen activator inhibitor type 1 (PAI-1). Rats and mice are commonly used animal species in pharmacological studies. However, very little is known about the fibrinolytic system of both species which prompted us to study the fibrinolytic system of rats and mice. At first we studied the ex vivo measured blood clot lysis. Blood clots prepared from 10% murine blood did not lyse spontaneously while blood clots prepared from 10% rat blood lysed for about 30% within 5 h. Addition of 2.5IU of human t-PA per ml diluted murine blood enhanced the lysis rate to a level comparable to rat blood. After i.v. administration of PAF, bradykinin, carbachol or adrenalin the lysis rate of both rat and murine blood clots was enhanced. The minimum dose of PAF, bradykinin, carbachol, or adrenalin which could induce a significant enhanced clot lysis was 0.06, 2.0, 0.8 and 0.8 μg/kg respectively for rats and 0.6, 2.0, 7.7 and 7.7 μg/kg respectively for mice. When no t-PA was added to mice blood clots these doses were not sufficient to induce a significant enhanced clot lysis; 3- to 10-fold higher doses were required. Addition of a FXIII inhibitor to diluted murine blood also increased the sensitivity for the used inducers. The doses which evoked an increased lysis rate under these conditions were 0.6, 0.6, 2.3 and 7.7 μg/kg respectively. No effects of FXIII inhibition were seen on lysis rates of rat blood clots. Secondly, acute t-PA release was studied in the perfused hindleg model with PAF, bradykinin, carbachol and adrenalin as inducers. Both rats and mice showed an acute t-PA release but differences in sensitivity were seen. The concentrations that induced half maximal release were 0.01, 1.2, 12 and 3 μM respectively for rats while for mice these values were 20, 3.7, 5.3 and 4.8 μM. The maximum amount of releasable t-PA in the perfusate was 26 U/kg for rats and 14 U/kg for mice measured using PAF and bradykinin respectively as inducer. Plasma PAI-1 levels were 1.7±0.2 U/ml and 3.7±0.3 U/ml for rats and mice respectively. Our results suggest that the high level of t-PA release and the lack of FXIII a mediated clot stabilization in rats contribute to the high rate of clot lysis compared to mice.