Drug-induced nephrotoxicity is a limiting factor in the therapeutic use of many drugs. Increased understanding of mechanisms involved in determining the change and extent of change in renal function has the potential to develop strategies to broaden the therapeutic index of nephrotoxic drugs. This proposal is a request for continued support for studies using amphotericin B as a model of a drug whose clinical utility as an anti- fungal and parasitic agent is curtailed by its nephrotoxic potential. We and others have shown that amphotericin B has a direct effect on vascular smooth muscle and mesangial cell contractibility via a calcium channel- mediated mechanism. The glomerular dynamic effects can be reduced not only by calcium channel blockade, but also by prior salt loading and by L- arginine and enhanced by L-N(G)-nitro-arginine methylester (L-NAME). These latter observations suggest the hypothesis that the release of nitric oxide (NO) from endothelial cells counteracts a direct contractile response to amphotericin B in smooth muscle cells. We propose to evaluate this hypothesis.
In Specific Aim 1, we will evaluate whether the glomerular response to acute amphotericin B is influenced by endogenous NO production in the rat. Pretreatment with L- and D-arginine, or asymmetrical dimethylarginine (ADMA), an endogenous competitive substrate for NO synthase, or L-NAME, an inhibitor of NO synthase activity, will be used to modulate NO synthase activity, and renal vascular responses will he assessed.
In Specific Aim 2, we will determine whether L-arginine confers a sustained renoprotective effect to chronic amphotericin B administration and whether such protection is additive to or the same as that provided by salt loading.
In Specific Aim 3, we will use vascular rings with or without endothelium, and cell cultures of endothelial cells, vascular smooth muscle cells and co-cultures of the two cell lines, to evaluate the hypothesis that amphotericin B interacts with free oxygen species to stimulate increases in intracellular calcium. In smooth muscle cells, this increase results in direct contraction. In endothelial cells, the increase in intracellular calcium results in increased NO production, which can then counteract the contractile response in smooth muscle cells via a cell:cell signalling mechanism involving cGMP.
In Specific Aim 4, we propose to undertake balanced, randomized, placebo controlled trial of L- arginine supplementation on the renal response to amphotericin B in man. Amphotericin B also induces changes in renal tubule function. In man, we have shown that salt supplementation helps prevent changes in glomerular function, but at the expense of increased renal potassium loss. We have also shown in rats that potassium deficit per se decreases NO production and enhances the renal tubule toxic response to amphotericin B. We now propose in Specific Aim 5 to undertake a balanced, randomized study to evaluate the hypothesis that renal responses to chronic amphotericin B are attenuated in patients with maintained and normal potassium status (augmented potassium supplement) in comparison to patients who have modest potassium depletion (conventional potassium supplement) due to augmented NO production. It is anticipated that an improved understanding of mechanisms involved in amphotericin B-induced nephrotoxicity can be used to develop strategies to enhance its safe, therapeutic use.
