On the interaction of bovine pancreatic trypsin inhibitor with maxi Ca(2+)-activated K+ channels. A model system for analysis of peptide-induced subconductance states.

Bovine pancreatic trypsin inhibitor (BPTI) is a 58-residue basic peptide that is a representative member of a widely distributed class of serine protease inhibitors known as Kunitz inhibitors. BPTI is also homologous to dendrotoxin peptides from mamba snake venom that have been characterized as inhibitors of various types of voltage-dependent K+ channels. In this study we compared the effect of DTX-I, a dendrotoxin peptide, and BPTI on large conductance Ca(2+)-activated K+ channels from rat skeletal muscle using planar bilayer methodology. As previously found for DTX-I (1990. Neuron. 2:141-148), BPTI induces the appearance of distinct subconductance events when present on the internal side of maxi K(Ca) channels. The single channel kinetics of substate formation follow the predictions of reversible binding of the peptide to a single site or class of sites with a Kd of 4.6 microM at 0 mV and 50 mM symmetrical KCl. The apparent association rate of BPTI binding decreases approximately 1,000-fold per 10-fold increase in ionic strength, suggestive of a strong electrostatic interaction between the basic peptide and negative surface charge in the vicinity of the binding site. The equilibrium Kd for BPTI and DTX-I is also voltage dependent, decreasing e-fold per 30 mV of depolarization. The unitary subconductance current produced by BPTI binding exhibits strong inward rectification in the presence of symmetrical KCl, corresponding to 15% of open channel current at +60 mV and 70% of open state at -40 mV. In competition experiments, the internal pore-blocking ions, Ba2+ and TEA+, readily block the substate with the same affinity as that for blocking the normal open state. These results suggest that BPTI does not bind near the inner mouth of the channel so as to directly interfere with cation entry to the channel. Rather, the mechanism of substate production appears to involve a conformational change that affects the energetics of K+ permeation.

Bovine pancreatic trypsin inhibitor as a probe of large conductance Ca(2+)-activated K+ channels at an internal site of interaction.

Bovine pancreatic trypsin inhibitor (BPTI) is a 58 residue protein whose binding to various serine proteases has been extensively studied by X-ray crystallography. We have found that BPTI also binds to an intracellular site associated with the large conductance Ca(2+)-activated K+ channel, as detected by the production of subconductance events in single channels incorporated into planar lipid bilayers. BPTI is highly homologous to a family of mamba snake dendrotoxin proteins that inhibit various K+ channels at an extracellular site. BPTI thus provides a useful model system to explore basic mechanisms underlying protein-channel interactions.

Mechanism of L-glucose, raffinose, and inulin transport across intact blood-brain barriers.

Brain capillary permeability-surface area products (PS) of hydrophilic solutes were evaluated in terms of a conventional two-compartment model. In rats whose blood-brain barrier (BBB) was presumed to be intact, metabolically inert carbohydrates with different molecular weights were injected in pairs to elucidate whether their transfer into the brain proceeds by diffusion through water- or lipid-filled channels or by vesicular transport. The distribution volume of 70 kDa dextran 10 min after intravenous injection was used as a measure of the residual volume of plasma in brain tissue after death. The two-compartment model yielded larger PS values for inulin and raffinose than for L-glucose, and the PS values of inulin and L-glucose were found to decrease as the labeling time was lengthened (10, 30, and 60 min). These observations were interpreted to mean that a rapidly equilibrating compartment was present between blood and brain, rendering the two-compartment model inadequate for computing true transfer rate constants. When multiple-time uptake data were reanalyzed using the three-compartment graphical analysis of Patlak, Blasberg, and Fenstermacher (J. Cereb. Blood Flow Metab. 3: 1-7, 1983), solutes of differing molecular size were found to enter the brain at approximately equal rates. This observation suggested that the predominant transport mechanism across an intact BBB is vesicular. Specifically, unidirectional transport is likely to be initiated by solute binding to the glycocalyx on the luminal surface of brain capillary endothelium. Apparently more inulin than L-glucose is absorbed, which may account for its slightly faster transfer across the BBB. We suggest that this adsorptive surface is the location of the rapidly equilibrating compartment on the plasma side of the BBB.

Transcapillary transport of solutes in the myocardium: effects of nitroglycerin, isoproterenol and histamine on permeability-surface area products of inulin and sucrose.

Three vasoactive drugs (nitroglycerin, isoproterenol and histamine) were examined for their effects on microsolute transport across capillary walls in the myocardium. Coronary arteries of the isolated rabbit heart were perfused at constant pressure with Tris-buffered Ringer's solution (pH = 7.4, 37 degrees C) with and without drug present in the perfusion fluid. A mixture of [3H]inulin and [14C]sucrose was injected into the left ventricular wall. From measured clearance rates, capillary permeability-surface area products (PS) (in milliliters per minute per 100 g) were computed for both solutes by the method of Gosselin and Stibitz (Pflügers Arch. 318: 85-98, 1970). Mean control PS values were 60.7 and 14.1 for sucrose and inulin, respectively. This computation required experimental determination of the myocardial volume of distribution (in milliliters per gram) for each reference solute. Values of myocardial volume of distribution obtained in the presence of nitroglycerin, isoproterenol and histamine did not differ significantly from controls. In paired clearance trials, isoproterenol and nitroglycerin significantly increased coronary flow, but neither drug influenced PS-inulin, PS-sucrose or the ratio PS inulin/PS sucrose. In contrast, histamine caused an apparently irreversible decrease in flow. Furthermore, in the presence of histamine, PS inulin/PS sucrose increased from 0.28 +/- 0.03 to 0.40 +/- 0.05 (P less than 0.003). This rise is consonant with a widening of diffusion channels between neighboring endothelial cells in the capillary wall. Thus, histamine (and presumably substances capable of histamine release) appears to increase myocardial permeability to microsolutes, in addition to its well known ability to enhance protein transport across postcapillary venules.