Intracellular calcium signals as well as modulations of the intracellular calcium concentration ([Ca²⁺]_i) regulate not only intracellular pathways but also complex neuronal networks like the respiratory network, which is located in the pre-Bötzingercomplex of the medulla oblongata. Intracellular [Ca²⁺]_i is regulated by the release from intracellular Ca²⁺-stores (e.g. mitochondria or ER via IP₃) and the influx from the extracellular space via voltage and/or receptor gated channels (e.g. L-typeCa²⁺-channel). Prazosin (an a1 receptor antagonist) and verapamil (an a1 receptor antagonist as well as a L-type calcium channel-blocker) which are medically used for the treatment of hypertension, are both modulators of calcium entry and for the release of calcium from the stores. Recordings of phrenic nerve activity of 21 day old wistar rats, using the in situ perfused working heart brainstem preparation, revealed that prazosin changed the frequency of the respiratory rhythm significantly, while verapamil in more than 80% of the experiments has no clear influence, but increases the respiratory frequency in the other 20%. In 66% of the experiments prazosin caused a two-phase-effect: a decrease of frequency of phrenic nerve discharges to nearly 50% was followed by an increase to 150%. This indicates that intracellular mechanisms exist, which compensate the effects initiated by blocking the a1-receptor in the respiratory network.