In this study of 29-month-old male Fischer and Sprague-Dawley rats, the tumor pheochromocytoma occured in seven of 20 Fischer rats (35%) and ten of 22 Sprague-Dawley rats (45%). Mean blood pressures and mean urinary vanillylmandelic acid levels of rats with pheochromocytomas did not vary significantly from rats with no tumours. Pheochromocytoma in our rats generally was non-functional or of minimal clinical significance. The content of chromaffin granules in tumor cells varied from abundant to not detectable, thus it was apparent that the absence of detectable chromaffin granules does not eliminate the diagnosis of pheochromocytoma.
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