We have compared the contractile responses of the isovolumic hearts of rat and guinea pig to a rise in the coronary perfusion pressure in the range 60-120 mmHg (Gregg's phenomenon). Left ventricular systolic pressure was lower in guinea pig hearts than in rat hearts at a low coronary perfusion pressure and increased markedly less at a higher perfusion pressure, despite a greater increase in coronary flow. The rise in left ventricular systolic pressure in the guinea pig hearts was entirely due to an increased left ventricular end-diastolic pressure, while left ventricular developed pressure did not increase. The wet weight of the hearts in situ was similar in both species, but after perfusion the guinea pig hearts gained significantly more fluid than the rat hearts (65% of the initial heart weight compared to 37%). The group of rat hearts perfused with a low external concentration of Ca2+ developed a similar left ventricular pressure to the guinea pig hearts and gained a similar amount of fluid (63%), but Gregg's phenomenon was the same as in rat hearts perfused with a normal concentration of Ca2+. The results suggest that the weak Gregg's phenomenon seen in guinea pig hearts can be attributed to factors other than myocardial edema and a lower left ventricular systolic pressure.
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