- 1. In guinea-pig trachealis Ca2+ are mobilized from different stores to produce the contractions to various agents. 2. Sr2+ can substitute for Ca2+ and maintain responses to KC1 but not responses to A23187 or agents acting via drug-receptor interaction. 3. Resting tone and responses to KC1 are supported by low affinity Ca2+. 4. Responses to the other agents are supported by high affinity Ca2+.