Fig. 2. cGMP is transported by tubules. (A) Effect of depolarisation on the cGMP transport ratio. Leucokinin (100 µmol l^–1) was used to depolarise the Malpighian tubule 30 min prior to the addition of cGMP (100 µmol l^–1; values are means ± s.e.m.; N10; ^***P<0.0001, Student's t-test). (B) cGMP-dependent kinase (cGK) activity in tubules is stimulated by fluid secreted from cGMP-treated Malpighian tubules. Data are expressed as mean (± s.e.m.) cGK activity, in pmol ATP min^–1 mg^–1 protein (N=3). cGK activity in tubule extracts is significantly stimulated by secreted drops from cGMP-treated tubules or by 100 µmol l^–1 cGMP, but not by secreted fluid from non-treated tubules (^***P<0.0001, Student's t-test).