Figure 3.

ADP ribosylation of RhoA by DC3B inhibits Ca²⁺ sensitization by phenylephrine, endothelin, and GTPγS, but not by phorbol ester. (A) Rabbit portal vein strips incubated for 48 h with or without DC3B were permeabilized with α-toxin (see MATERIALS AND METHODS). Isometric tension was measured at constant [Ca²⁺] (pCa 6.5), followed by stimulation with phenylephrine (100 μM) plus GTP (10 μM) or endothelin (100 nM) plus GTP (10 μM). GTPγS (50 μM) was applied on the plateau phase of PE plus GTP-induced contraction. Traces shown are representative of phenylephrine plus GTP-induced Ca²⁺ sensitization, followed by stimulation with GTPγS. G1; calcium-free solution containing 1 mM EGTA, CaG; pCa 4.5, 10 mM EGTA buffered. (B) Summary of the effect of DC3B on the phenylephrine-, endothelin- and GTPγS-induced, and lack of effect on phorbol ester-induced, Ca²⁺ sensitization. PE, phenylephrine (100 μM); ET, endothelin (100 nM); PDBu, phorbol-12,13-dibutyrate (1 μM). Phenylephrine-, endothelin-, and total (PE + GTP + GTPγS)-induced Ca²⁺-sensitization were significantly inhibited by DC3B (48 h).