Figure 7. The effects of reducing external calcium on the permeability response to histamine H₂ receptor activation.

A, dimaprit (5 μm) was applied to the abluminal surface of a single occluded microvessel (diameter, 16 μm) that was superfused with a solution containing the usual 2.5 mm Ca²⁺, and resulted in a dye loss that increased P_LY from 0.13 × 10⁻⁶ to 1.53 × 10⁻⁶ cm s⁻¹. When the dimaprit application was repeated about 90 s after the first application, there was a similar full response. B, the same microvessel with repeated dimaprit applications but with a nominally calcium-free superfusing solution. The permeability increases were only transient, and lasted for 48 s with the first dimaprit application and 18 s with the second. The P_LY values are given below the relevant parts of the curves (expressed as 10⁻⁶ cm s⁻¹). C, summary of P_LY changes measured 60 s after dimaprit application in 4 vessels from 4 rats (diameter, 12–16 μm); similar experiments to those shown above. The mean P_LY increased from (0.86 ± 0.187) × 10⁻⁶ cm s⁻¹ before, to (2.84 ± 0.453) × 10⁻⁶ cm s⁻¹ 60 s after the beginning of the application of 5 μm dimaprit with the normal calcium-containing superfusing solution (□, control; ▪, dimaprit). When a nominally zero calcium superfusate was used, P_LY hardly changed at all (from (1.07 ± 0.491) × 10⁻⁶ to (1.39 ± 0.797) × 10⁻⁶ cm s⁻¹; ^, control; •, dimaprit). The difference in the responses was statistically significant (P < 0.005; Student's paired t test).