Fig. 1. Electrochemical gradients and ion activities in (A) unstimulated and (B) 5-HT-stimulated Malpighian tubules of Rhodnius prolixus. Values for pH_bath, pH_i, pH_lumen and luminal a_K, a_Cl and a_Na are presented as means ± s.e.m. for (N) tubules. Values in B were recorded 30 min after stimulation with 10^–6 mol l^–1 5-HT. Intracellular and bath activities for Na⁺, K⁺ and Cl^– (italicised) and electrochemical gradients for Na⁺, K⁺ and Cl^– across the basolateral membrane are taken from Ianowski et al. (Ianowski et al., 2002).