TABLE 1.

Inhibitor constant (K_i) for various cardiotonic steroids at pH 6.5, 7.5, and 8.5 after a 1-h preincubation at 23 °C (mean ± S.D., n = 3)

CTS	Ki			Steroid –OH substitutions
	pH 6.5	pH 7.5	pH 8.5
	μm
Ouabagenin	0.406 ± 0.077	2.28 ± 0.20	32.10 ± 0.78	C1β, C5β, C11α, C19β
Ouabain	0.086 ± 0.011	0.091 ± 0.011	1.18 ± 0.27	C1β, C5β, C11α, C19β
Digitoxigenin	0.132 ± 0.014	0.163 ± 0.018	0.373 ± 0.038
Digitoxin	0.183 ± 0.001	0.176 ± 0.012	0.404 ± 0.028
Digoxigenin	0.136 ± 0.006	0.195 ± 0.001	1.56 ± 0.34	C12β
Digoxigenin monodigitoxide	0.107 ± 0.08	0.122 ± 0.012	0.582 ± 0.64	C12β
Digoxigenin didigitoxide	0.101 ± 0.004	0.120 ± 0.006	0.614 ± 0.017	C12β
Digoxin	0.102 ± 0.008	0.131 ± 0.020	1.03 ± 0.04	C12β
Digoxigenin tetradigitoxide	0.078 ± 0.019	0.204 ± 0.014	1.46 ± 0.19	C12β
Gitoxigenin	0.375 ± 0.055	0.428 ± 0.035	6.28 ± 0.69	C16β
Gitoxin	0.144 ± 0.007	0.156 ± 0.012	0.423 ± 0.057	C16β
Evomonoside	0.100 ± 0.014	0.106 ± 0.009	0.174 ± 0.002
Strophanthidin	0.139 ± 0.014	0.173 ± 0.038	0.549 ± 0.003	C5β
Strophanthidol	0.170 ± 0.021	0.166 ± 0.025	1.34 ± 0.14	C5β, C19β
Bufalin	0.140 ± 0.017	0.134 ± 0.001	0.271 ± 0.018
Gamabufotalin	0.154 ± 0.013	0.159 ± 0.022	0.371 ± 0.021	C11α