Contribution of the endothelium to transcapillary insulin transport in rat isolated perfused hearts.

Capillary endothelial cells are thought to limit the transport of insulin from the vascular to the interstitial space, resulting in attenuated hormonal action at target sites. This study examined the contribution of endothelial cells to the regulation of transcapillary insulin transport in rat hearts in vitro. Hearts were perfused with a protein-free buffer that resulted in the generation of a substantial amount of interstitial fluid (transudate) that was collected at the surface of ventricles. Insulin (0.05-1 U/l) was added to the perfusate, and its transfer kinetics to and clearance from the interstitium were analyzed from insulin measurements in transudate of hearts with intact or collagenase-disrupted endothelium. In endothelium-intact hearts (n = 5-8), the steady-state insulin concentration in transudate was 29 +/- 4, 30 +/- 2, 53 +/- 1, 103 +/- 6, and 97 +/- 4% of perfusate concentrations at 0.05, 0.1, 0.2, 0.5, and 1 U/l insulin, respectively. The corresponding apparent rate constants for transport (k(in)) increased from 0.03/min to -0.27/min, indicating a nonsaturable transport process. The transport rate for [3H]insulin (1.2 nmol/l; n = 5) was identical to an equimolar concentration of insulin (0.2 U/l), strongly indicating the same mode of transport. In endothelium-disrupted hearts (n = 3-5), the same perfusate/transudate concentration ratios were observed--that is, a gradient at low insulin concentrations (0.05-0.2 U/l) and complete equilibration at higher insulin concentrations, suggesting a contribution of reabsorption processes back into the vascular space in the generation of the gradient. Finally, inhibition of endothelial nitric oxide (NO) formation by NG-nitro-L-arginine (200 micromol/l) affected neither k(in) nor the extent of transendothelial insulin transport in the presence of an intact endothelium. We concluded that 1) capillary endothelial cells affect the transcapillary transport of insulin by slowing the transfer to the interstitium, 2) insulin is transported by a bidirectional convective transport rather than by a saturable receptor-mediated mechanism, and 3) endothelium-derived NO is without effect on transcapillary insulin transport in this model.