ENDOGENOUS EGF AS A POTENTIAL RENOTROPHIC FACTOR IN ISCHEMIA-INDUCED ACUTE-RENAL-FAILURE

The time course for the increases in soluble renal epidermal growth fa ctor (EGF) after ischemia has been established. These elevated levels of EGF have been compared with the degree of tissue injury as well as the extent of cell proliferation in the recovering tissue. Levels of s oluble immunoreactive EGF (irEGF) in control animals were 9.74 +/- 1.1 ng/g wet wt (n = 4-8 for all values) and rose to 83.9 +/- 30 ng/g wit hin 12 h after injury. Soluble irEGF content peaked at 88.8 +/- 15 ng/ g at 24 h postinjury and returned to control values by 72 h. We previo usly reported that trypsin digestion of crude renal membranes (CRM) ge nerates rat EGF that is indistinguishable from that isolated from the submandibular gland. Initial levels of trypsin-releasable membrane-ass ociated irEGF were 439 +/- 26 ng/g. These levels fell to 46.6 +/- 9.6 ng/g at 48 h after injury. The total renal EGF demonstrated an 80% dec line 48 h after injury but returned to 50% of the initial values after 72 h representing significant new synthesis of EGF-containing protein s between 48 and 72 h postinjury. Immunohistochemical staining of kidn ey paraffin sections for EGF immunoreactivity demonstrated staining in tensities that paralleled the amount of irEGF in the trypsin-digested CRM fraction, suggesting that the membrane-associated irEGF is the pre dominant form detected by this technique. Regenerative hyperplasia sub sequent to tubular insult was monitored by immunostaining nuclei of S phase cells after pulse labeling with the thymidine analogue 5-bromo-2 '-deoxyuridine. Cell proliferation was particularly prominent in the o uter stripe of outer medulla of kidneys exposed to ischemia and reache d a maximum 19-fold higher than the baseline value) 48 h after reperfu sion. Renal cell turnover returned to control values by day 7. The obs ervation that the peak in soluble EGF levels (24 h) precedes the peak in tubular regeneration (48 h) by 24 h is consistent with the hypothes is that EGF is one of the mitogenic signals triggering regenerative hy perplasia after renal injury.