Proteinuria in normal dogs can be produced at will by parenteral injections of dog plasma.
As the plasma injections are continued the plasma protein concentration rises and at some point protein begins to appear in the urine. The level of plasma protein concentration at which proteinuria appears in normal dogs ranges from 9.6 to 10.4 gm. per cent. This may be termed the renal threshold for proteinuria. Repeat experiments in the same dog show threshold levels to be practically identical.
An interval of days (4 to 26 days) has been noted between the start of plasma protein injections and the appearance of the proteinuria. Larger doses of plasma shorten this interval and the critical plasma protein level is attained sooner.
Considerable amounts of protein may appear in the urine—298 gm. protein during a 52 day period in one instance studied—yet the urine clears in 1 to 4 days after cessation of protein injections. Autopsy shows undamaged kidneys.
Maximal levels of plasma protein concentration range from 10.0 to 11.5 gm. per cent. The highest levels are usually associated with maximal output of protein in the urine.
It seems clear that plasma proteins readily pass cell barriers (or membranes) within the body, including the endothelium and epithelium of the renal glomerulus.