Recently identified imidazoline receptors of the first type (I(1)Rs) on the cardiomyocyte's sarcolemma open a new field in calcium signaling research. In particular, it is interesting to investigate their functional interaction with other well-known systems, such as beta-adrenergic receptors. Here we investigated the effects of I(1)Rs activation on L-type voltage-gated Ca2+-currents under catecholaminergic stress induced by the application of beta-agonist, isoproterenol. Pharmacological agonist of I(1)Rs (I-1-agonist), rilmenidine, and the putative endogenous I-1-ligand, agmatine, have been shown to effectively reduce Ca2+-currents potentiated by isoproterenol. Inhibitory analysis shows that the ability to suppress voltage-gated Ca2+-currents by rilmenidine and agmatine is fully preserved in the presence of the protein kinase A blocker (PKA), which indicates a PKA-independent mechanism of their action. The blockade of NO synthase isoforms with 7NI does not affect the intrinsic effects of agmatine and rilmenidine, which suggests NO-independent signaling pathways triggered by I(1)Rs. A nonspecific serineithreonine protein phosphatase (STPP) inhibitor, calyculin A, abrogates effects of rilmenidine or agmatine on the isoproterenol-induced Ca2+-currents. Direct measurements of phosphatase activity in the myocardial tissues showed that activation of the I(1)Rs leads to stimulation of STPP, which could be responsible for the I-1-agonist influences. Obtained data clarify peripheral effects that occur during activation of the I(1)Rs under endogenous catecholaminergic stress, and can be used in clinical practice for more precise control of heart contractility in some cardiovascular pathologies. (C) 2019 Elsevier Inc. All rights reserved.