Fig. 4. (A) A 24 hour exposure to the -adrenoceptor agonist phenylephrine has no significant effect on the percentage of apoptotic hemocytes, whereas (B) a 24 hour exposure to the ß-adrenoceptor agonist isoproterenol mimicks the apoptosis-inducing effect of NA. (C) Addition of the -adrenoceptor antagonist prazosin 30 minutes prior to the addition of 1 µM NA has no significant effect on the percentage of apoptotic cells whereas (D) addition of the ß-adrenoceptor antagonist propranolol blocks the apoptosis-inducing effect of NA. Data are means and standard errors of three replicate experiments. Asterisks denote significant (P<0.01) differences from samples incubated in the absence of adrenoceptor agonist (A,B) or in the presence of 1 µM NA alone (C,D). Bas (basal level) indicates the percentage of apoptotic cells in samples incubated in the absence of drugs.